The Phenomenon of Treatment Dropout, Reasons and Moderators in Acceptance and Commitment Therapy and Other Active Treatments: A Meta-Analytic Review


Research Article

The Phenomenon of Treatment Dropout, Reasons and
Moderators in Acceptance and Commitment Therapy and
Other Active Treatments: A Meta-Analytic Review

Maria Karekla a, Pinelopi Konstantinou a, Myria Ioannou a, Ioannis Kareklas b,

Andrew T. Gloster c

[a] University of Cyprus, Nicosia, Cyprus. [b] University at Albany, New York, NY, USA. [c] University of Basel, Basel,
Switzerland.

Clinical Psychology in Europe, 2019, Vol. 1(3), Article e33058, https://doi.org/10.32872/cpe.v1i3.33058

Received: 2019-01-13 • Accepted: 2019-05-09 • Published (VoR): 2019-09-20

Handling Editor: Cornelia Weise, Philipps-University of Marburg, Marburg, Germany

Corresponding Author: Maria Karekla, Department of Psychology, University of Cyprus, P.O. Box 20537, Nicosia
1678, Cyprus. Tel: 357 22 892100; Fax: 357 22 892071. E-mail: mkarekla@ucy.ac.cy

Abstract
Background: Treatment dropout is one of the most crucial issues that a therapist has to face on a
daily basis. The negative effects of premature termination impact the client who is usually found to
demonstrate poorer treatment outcomes. This meta-analysis reviewed and systematically examined
dropout effects of Acceptance and Commitment Therapy (ACT) as compared to other active
treatments. The goals of this study were to compare treatment dropout rates and dropout reasons,
examine the influence of demographic variables and identify possible therapy moderators
associated with dropout.
Method: The current meta-analysis reviewed 76 studies of ACT reporting dropout rates for
various psychological and health-related conditions.
Results: Across reviewed studies (N = 76), the overall weighted mean dropout rate was 17.95%
(ACT = 17.35% vs. comparison conditions = 18.62%). Type of disorder, recruitment setting and
therapists’ experience level were significant moderators of dropout. The most frequently reported
reasons for dropout from ACT were lost contact, personal and transportation difficulties, whereas
for comparative treatments they were lost contact, therapy factors and time demands.
Conclusion: Given that most moderators of influence are not amenable to direct changes by
clinicians, mediation variables should also be explored. Overall, results suggest that ACT appears to
present some benefits in dropout rates for specific disorders, settings and therapists.

Keywords
acceptance and commitment therapy, dropout, attrition, meta-analysis, premature termination

This is an open access article distributed under the terms of the Creative Commons Attribution
4.0 International License, CC BY 4.0, which permits unrestricted use, distribution, and
reproduction, provided the original work is properly cited.

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Highlights
• There was no difference in dropout rate between ACT and control conditions

(17.35% vs.18.62%). Significant moderators were client disorder, therapists’
experience level and recruitment and setting.

• Comparison condition frequently reported therapy related dropout factors,
suggesting that ACT may be a more acceptable option.

Acceptance and Commitment Therapy (ACT), is a so-called third wave Cognitive Behav‐
ior Therapy (CBT) and has been applied successfully to treat numerous problems and dis‐
orders (Hayes, Luoma, Bond, Masuda, & Lillis, 2006; Ruiz, 2012). ACT helps clients
choose to do what takes them closer to their goals (especially when dealing with prob‐
lematic thoughts and emotions) rather than aiming to reduce symptoms directly (Hayes,
Hayes, Strosahl, & Wilson, 2012). The focus is placed on the experience of the person and
the function of any behavior rather than on actions being carried out based on the literal
content of a belief (Hayes et al., 2006). The overall aim of treatment is to increase psycho‐
logical flexibility or the ability to fully contact the present moment, choosing to act gui‐
ded by the person’s values in the context at hand (Fletcher & Hayes, 2005). Most existing
reviews and meta-analyses of ACT support that it is at least as equally effective as tradi‐
tional Cognitive Behavioral Therapy (tCBT) on indices of symptom reduction and more
effective than other comparison conditions (A-Tjak et al., 2015; Powers, Vörding, &
Emmelkamp, 2009; Ruiz, 2012).

Treatment outcomes and effectiveness, however, are affected not only by the specific
treatment provided but also by other factors such as premature termination/dropout or
non-completion of the specified interventions (Barrett, Chua, Crits-Christoph, Gibbons,
& Thompson, 2008). Premature therapy termination or treatment dropout is a significant
problem or obstacle limiting the effectiveness of any therapeutic approach and results in
detrimental outcomes in patients (Barrett et al., 2008; Wierzbicki & Pekarik, 1993). Un‐
fortunately, there is no consensus definition about what constitutes treatment dropout.
General definitions of dropout include: termination of the intervention prior to the pa‐
tient recovering from the problem(s) for which treatment was initially sought (Hatchett
& Park, 2003; Swift, Callahan, & Levine, 2009), or treatment termination without the
agreement of the therapist and before the scheduled end point (Stone & Rutan, 1984).
However, in research protocols premature termination may be considered as missing a
number of pre-arranged sessions (e.g., four consecutive weeks in DBT; Linehan, 1993) ir‐
respective of the patient’s recovery status.

Reviews and meta-analyses of this phenomenon focus on examining first the rates of
dropout and, secondly, variables associated with its occurrence. Swift and Greenberg
(2012) examined dropout definition as a moderator of dropout rates and found higher
rates when the therapist judged dropout status, compared to other definitions. This was

Treatment Dropout in ACT 2

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one of the first comprehensive reviews of the dropout phenomenon encompassing vari‐
ous forms of psychotherapy and concluded that 1 in 5 clients drop out prematurely, a rate
somewhat lower than previous reviews (e.g., Wierzbicki & Pekarik, 1993). Client diagno‐
sis, age, education, gender, marital status, time-limitations of treatment, use of manual or
protocol, treatment setting, providers’ level of experience, dropout definition, study type
and search strategy were found to be significant moderators of dropout. However, this
meta-analysis did not include studies of third wave psychological treatments, like ACT.
Moreover, it focused only on adult populations and did not include substance or alcohol
abuse disorders, health-related problems (e.g., weight management, emotional burnout),
and self-help interventions. Finally, it focused on providing a broad analysis of premature
discontinuation in psychological treatments and not on reasons for dropout.

This study aims to examine the dropout phenomenon in ACT (compared to other ac‐
tive interventions) because of ACT’s emphasis on connecting clients with their deeply
held values and through this process to motivate them towards behavior change. If ACT
is successful in mobilizing individuals via the treatment process, we expect that this
would prevent premature termination and thus ACT would result in lower dropout rates
compared to other interventions. To date, only one meta-analysis on dropout has inclu‐
ded ACT (Ong, Lee, & Twohig, 2018). This study found that only therapist experience sig‐
nificantly predicted dropout, specifically that when ACT was provided by master’s level
therapists higher dropout rates were observed, compared to other levels of therapists’ ex‐
perience (e.g., PhD level psychologist, MD physician, graduate student). However, under‐
standing dropout in ACT can be further facilitated in four important ways. First, inclu‐
sion of variables found to predict dropout in previous meta-analyses (e.g., gender, race,
marital status, employment and years of education) will allow for comparison across
studies and methods (e.g., Swift & Greenberg, 2012). Second, inclusion of variables that
assess how the therapy and study were implemented (e.g., length of intervention, hours
of intervention, setting, definition of dropout, study type, year of publication and region)
can reveal clues as to how interventions can actively minimize dropout. Third, testing the
reasons and timing of dropouts provides hypotheses for researchers and therapists to ac‐
tively intervene to prevent this phenomenon of paramount clinical significance. Finally,
some methodological details regarding comparison groups are worthy of reexamination.
For example, we believe that including waitlist control conditions in the comparisons
may bias the dropout findings in favor of waitlist control. This is because people on the
waitlist are fundamentally different to patients in a control condition. In the waitlist, pa‐
tients usually maintain hope that things will get better once the treatment begins and are
not motivated to actively change during the waiting period. When clients do drop out
during this period, by definition it has nothing to do with the active treatment. Thus a
cleaner comparison of treatment dropout should be carried out between different active
treatments (including active controls). Towards this goal, comparative conditions should
be other active interventions.

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A minority of patients may drop out because they improved or met their goals; how‐
ever numerous individuals drop out because of a problem with the treatment or thera‐
pists or for other unforeseen circumstances. Specifically, proposed problems or reasons
associated with increased dropout rates include: client demographic characteristics (e.g.,
younger age, female gender, low socioeconomic status; Wierzbicki & Pekarik, 1993); type
of psychopathological difficulties (e.g., eating or personality disorders); therapist charac‐
teristics (e.g., provider in training); therapy setting (e.g., university-based clinics); and
specific factors (e.g., non-time limited therapy), and environmental variables or acute
problems that take greater priority (Bados, Balaguer, & Saldaña, 2007; Roe, Dekel, Harel,
& Fennig, 2006; Swift & Greenberg, 2012).

Researching these reasons is difficult as variables and methods vary widely depend‐
ing on the study and its focus, the population studied, the treatment setting or the treat‐
ment offered (Roe et al., 2006; Todd, Deane, & Bragdon, 2003). However, there is an
agreement that certain common reasons account for dropout. These include: lack of im‐
provement or accomplishment of goals, dissatisfaction with the treatment, and environ‐
mental obstacles and constraints (Hunsley, Aubry, Verstervelt, & Vito, 1999; Pekarik,
1992; Roe et al., 2006; Todd et al., 2003). In addition to common factors, clients report spe‐
cific reasons for discontinuation, including: external circumstantial problems and difficul‐
ties (e.g., transportation problems, moving away, timetables), illness and new responsibil‐
ities, improvement due to therapy, satisfactory achievement of treatment goals, high
treatment costs, dissatisfaction with the therapist and psychotherapy, no need for serv‐
ices and need for independence and trying to solve problems without therapy (Bados et
al., 2007; Roe et al., 2006; Todd et al., 2003).

Interestingly, very little attention has been given to the timing during treatment when
premature termination occurs and most studies do not even report this information.
Some have proposed that the first two sessions are critical for premature termination,
given that most dropouts (70%) occur at this point, making it a critical period to success‐
fully engage the client in treatment (Olfson, Mojtabai, Sampson, Hwang, & Kessler, 2009).
Karekla (2004) observed that in a comparison trial of CBT vs. ACT for panic disorder,
though dropout rates between the two approaches were similar, most individuals who
dropped out in the CBT condition did so immediately after the introduction of exposure.
Such a pattern was not evident for the ACT group, where individuals dropped out at var‐
ious times during treatment unrelated to specific treatment components. It was conclu‐
ded that ACT might present an advantage over CBT not in terms of symptom reduction
but that it may better prepare individuals to engage in exposure of previously avoided
internal and external events and in dealing with the dropout problem. To date, none of
the reported reviews or meta-analyses of ACT have examined in depth dropout, dropout
reasons, extensive list of moderators, and compared to active treatments.

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Current Study
The purpose of this study is to examine dropout rates, dropout reasons’ associated fac‐
tors, and potential moderators of dropout, in ACT compared to active comparison condi‐
tions. The goals of this study were to: (i) compare treatment dropout rate and timing be‐
tween ACT and other active treatments; (ii) examine the influence of demographic varia‐
bles such as age, gender, treatment setting, race, education, duration of treatment, ethnic‐
ity and diagnosis on dropout; (iii) identify possible therapy-associated moderators of
dropout; and (iv) examine timing and possible reasons for dropout.

M e t h o d
This review was registered in the International Prospective Register of Systematic Re‐
views (see Supplementary Materials).

Literature Search
The literature search was conducted using the computerized literature databases Google
Scholar, EBSCOhost (Academic Search Ultimate, Medline, Psychology and Behavioral Sci‐
ences Collection, PsycARTICLES, PsychInfo, OpenDissertations) and Science Direct (until
June 2018) with the following keywords based on title: “Acceptance and Commitment
Training”, “Acceptance-based behavior therapy”, “ACT-based”, “Experiential avoidance”,
“Psychological flexibility”, “RFT-based”, “CBS-based”, “Third wave CBT therapies” “Ac‐
ceptance and Commitment Therapy”, and “ACT”; alone first and then also combined with
the terms “drop out” or “dropout” or “discontinuation” or “outcome” or “premature termi‐
nation” or “termination”. The reference lists of all identified articles were examined for
additional potentially eligible studies, as well as existing meta-analyses and reviews. A
request for unpublished studies was sent to the Acceptance and Commitment Therapy
(ACT) listserv (https://contextualscience.org/emailing_lists), as well as to the primary or
secondary authors of identified articles, via email.

Eligibility Criteria
Identification and selection of the included studies was performed by the second author,
a clinical psychology doctoral student, who was first trained and instructed in the proce‐
dure of conducting meta-analysis by the first and last authors. Everything was checked
by the first author. The last three authors all have experience in meta-analysis and served
to check all steps taken in the process of this study.

This study includes all published and unpublished (e.g., dissertation) Acceptance and
Commitment Therapy studies that included dropout information and met the following
criteria. Studies were included if they: (1) were in English, (2) reported dropout rates after
beginning psychotherapy or reported no dropouts (i.e., all participants completed treat‐

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ment), and (3) used an active comparison condition. Studies were excluded if: (a) data ori‐
ginated from the same sample as another included study (so as to avoid violating the 
meta-analytic assumption of data independence); (b) information to calculate effect sizes 
was lacking and contact with authors was not possible; and (c) case studies.

The literature search resulted in 4399 articles in total. After screening the titles and 
abstracts, and following the examination of the full papers, 76 studies met all aforemen‐
tioned inclusion criteria and were retained for analysis (see Figure 1 for procedure de‐
tails). Based on the Rosenthal’s suggestions for computing the fail-safe n, it was found 
that the total Z value was -1.181 and the number of missing studies we would need to 
retrieve and incorporate to result in a non-significant p-value was 147 studies (see also 
Borenstein, Hedges, Higgins, & Rothstein, 2009).

Coding Procedures
Treatment dropout was defined as the percentage of patients who began treatment, but 
according to the author(s) dropped out prematurely, thus utilizing the author(s)’ defini‐
tion. For reliability and validity purposes we included only studies that reported dropout 
rates during treatment and not prior to treatment initiation.

Participant, therapist, treatment and study characteristics were coded (see Table 1 for 
details about coding of each of the variables). Eight participant characteristics included: 
client disorder, gender, age, race, marital status, employment, years of education and pop‐
ulation. Eight treatment variables were comparison condition, treatment status, length of 
intervention, length of intervention in sessions, hours of intervention, format of treat‐
ment, treatment setting and description of treatment setting (as per Swift & Greenberg, 
2012). Two treatment provider variables regarding experience level: 1) experience level of 
ACT therapists, and 2) experience level of therapists in comparison groups. Finally, four 
study variables included: definition of dropout, study type, year of publication and re‐
gion.
   The second and third authors coded all variables separately and these were checked for 
accuracy by the first authors. There was a 95% agreement rate between coders with disa‐
greements resolved via a consensus among the authors (for further coding details contact 
the authors).

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Figure 1. Flow chart of information from identification to inclusion of studies in this review.

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Table 1

Details Regarding the Coding of Each of the Variables

Participant characteristics
Client disorder Anxiety disorder (including social phobia, public speaking anxiety, generalized anxiety disorder

and obsessive compulsive disorder), depression, substance abuse or dependence, chronic pain
(including fibromyalgia, osteoarthritis and headaches), eating pathology/disorder (including
diabetes, obesity, weight problems and eating disorders), health conditions and chronic illnesses
(i.e. Parkinson’s disease, multiple sclerosis, brain injury, cancer and HIV), smoking, other health
problems (stress, distress, physical activity, tinnitus, procrastination and sickness absence) and
severe psychopathology (including borderline personality disorder, treatment resistant and
psychosis)

Gender Percentage of female participants in each study
Age Average age in years of participants in each sample
Race Percentage of White (including Caucasian, Australian and European), Black (including African

American) and other (Hispanic, Latino, Asian American/ Pacific Islander, Native American,
Alaskan American and American Indian/ Alaskan native)

Marital status Percentage of participants who were single (non-married, never married, divorced, separated or
widowed) vs. married (cohabiting, living with partner/spouse/family or in a relationship)

Employment Percentage of participants who were working, either full-time or part-time
Years of education Participants’ average number of completed education years in each study. In cases where the

mean number of education in years was not provided, we calculated this based on the data
reported.

Population Adults or children and adolescents

Treatment variables
Comparison condition CBT, Treatment as Usual (TAU; studies in which TAU consisted of only administrating medication

were coded as medication only), medication only (i.e., Medication Treatment as Usual plus
Enhanced Assessment and Monitoring, Recommended Pharmacological Treatment, Specialty
Medical Management, Methadone maintenance, Selective Serotonin Reuptake Inhibitors, Medical
Treatment as Usual, Nicotine Replacement Treatment and Bupropion Regimen), other active
treatment (i.e., Narcotics Anonymous, Applied Behavior Analysis, smokefree.gov, online
discussion forum, usual care, counseling services, Workplace Dialogue Intervention, Present-
Centered Therapy, physical exercise, Drug Counseling, Tinnitus Retraining Therapy and
Expressive writing), component of CBT (including Progressive Relaxation Training, Systematic
Desensitization, Applied Relaxation, Cognitive Therapy, Stress Inoculation Training, Relaxation
Training) and education only (education, Befriending, Pedometer-based walking program)

Treatment status Providing any treatment/training to the comparison condition or not
Length of intervention Total length of treatment in weeks (in cases where months were reported, each month was

calculated to equal 4 weeks)
Length of intervention
in sessions

Total number of treatment sessions

Hours of intervention The overall duration of intervention in hours
Format of treatment Individual, self-help (including web-based and online format), group, or combination (group &

individual)
Treatment setting Outpatient, inpatient or self-help (including web-based and online format)
Description of
treatment setting

University affiliated clinic (psychology department training clinic and university counseling
center), outpatient clinic affiliated with a hospital or medical school, public/community outpatient
clinic, research/specialty clinic, private outpatient clinic/practice, therapy took place at
participant’s home (i.e., web-based/online intervention or self-help) and inpatient or residential
treatment

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Treatment provider variables
Experience level of
ACT therapists

Master level therapists or doctoral students/interns/residents, doctoral level or licensed therapists,
mix of doctoral level, student trainees, and others (e.g., licensed clinicians, psychiatrists, social
workers, psychiatric nurses), no therapists (i.e., for online/web-based or self-help formats), mix of
different psychologist levels and non-psychologists (e.g. drug staff, alcohol counselor, physician,
psycho-pharmacologist)

Experience level of
therapists in
comparison groups.

Master level therapists or doctoral students/interns/residents, doctoral level or licensed therapists,
mix of doctoral level, student trainees, and others (e.g., licensed clinicians, psychiatrists, social
workers, psychiatric nurses), no therapists (i.e. it was applicable for online/web-based or self-help
formats), mix of different psychologist levels, psychiatrists and non-psychologists (e.g. drug staff,
alcohol counselor, physician, psycho-pharmacologist).

Study variables
Definition of dropout Failed to complete treatment/discontinued treatment/left before treatment end, or refused to

return to treatment, failed to attend all sessions, failed to submit pre and post treatment data and
attended less than or equal to either: 25-40%, 50-75% or 76-90% of total sessions

Study type Efficacy (i.e., studies that emphasize internal validity) or effectiveness (i.e., emphasize external
validity of the experimental design). If the study type was not specifically reported, efficacy was
coded as studies utilizing: (a) strict exclusion criteria, (b) careful pre-selection of clients, (c)
treatment following a strict protocol and was more controlled than effectiveness studies, (d)
randomization of participants to treatments, and/or (e) therapists receiving training before and
supervision during the study

Year of publication
Region In which each study was conducted

Data Analysis
First, the dropout rate for each study condition (ACT vs. comparison group) was calcula‐
ted (i.e., the total number of patients who dropped out of each treatment group, out of
the total number of patients included in each group). Then, the weighted average drop‐
out rate (i.e., weighted dropout rate for each study condition based on the total number
of patients included in the study) was computed for each of the 76 included studies. The
number of participants dropped from each group was included in the Comprehensive
Meta-Analysis software (CMA; version 2.0, Biostat, Englewood, NJ), along with the sam‐
ple size of each group (treatment and comparison). Odds ratio was then computed. Odds
ratios higher than 1 suggest that dropout rates are higher in the intervention versus the
comparison condition (i.e., comparison group is better).

Random-effects models were used to estimate the effect size of rate ratio in the inclu‐
ded studies, as the assumptions of random-effect models suggests that study characteris‐
tics influence the true effect of treatments, and that sampling error varies between stud‐
ies (DerSimonian & Kacker, 2007). The Q statistic and the I2 statistic were calculated. Ran‐
dom-effects models are considered appropriate when there is significant heterogeneity (p
< .05) according to the Q index, and when heterogeneity is high (>75%) based on the I 2
index.

First, an unconditioned model without having any predictors or moderators was cal‐
culated using CMA, in order to detect the general rate ratio of dropouts between treat‐

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ment and comparison conditions. In order to examine if the results of the general model
were subject to biases related to the publication of studies with favorable outcomes, pub‐
lication bias was investigated by assessing the asymmetries evident in a funnel plot, with
the Egger’s regression test (Egger, Smith, Schneider, & Minder, 1997) and the Begg and
Mazumdar test (Kendall’s statistic). A stratified subgroup analyses was then run in order
to test the moderating role of categorical study characteristics and meta-regression anal‐
yses to test the moderating role of continuous study characteristics. Q statistic was calcu‐
lated for the subgroup analyses, in order to examine if the differences detected between
the mean effect sizes of the groups of studies with a particular characteristic were signifi‐
cant. The meta-regression analyses were computed using a general mixed-effects meth‐
od-of-moments (Kacker, 2004) estimate for the inter-study variance τ2 (DerSimonian &
Kacker, 2007).

R e s u l t s

Characteristics of Reviewed Studies
All identified studies were included in the meta-analysis; no structured qualitative assess‐
ment of the reviewed articles was performed. The large majority of included studies em‐
ployed a randomized controlled trial design, or at least a controlled trial design. This sug‐
gests that all studies are at least of a moderate methodological quality (Petrisor &
Bhandari, 2007), and attempted to compare ACT to an active treatment comparison con‐
dition.

See Table 2 for characteristics of included studies. Most studies dealt with the treat‐
ment of anxiety (n = 14, 18.4%) and chronic pain (n = 14, 18.4%); and targeted adults (n =
73, 96.1%) using a group treatment format (n = 34, 44.7%). ACT was compared mostly
with TAU (n = 17, 22.4%) and CBT (n = 17, 22.4%). Most studies were delivered in an out‐
patient setting (n = 60, 78.9%) and participants were most frequently recruited via com‐
munity advertisements (n = 21, 27.6%). Treatment in ACT groups was delivered mostly by
psychologists of various training levels (n = 19, 25%) and licensed or doctoral level psy‐
chologists (n = 15, 19.7%). In comparison group treatment was provided mostly by a mix
of doctoral level, student trainees and others (n = 15, 19.7%) and a mix of psychologists
with different training levels (n = 13, 17.1%). The most frequent definition of dropout was
“failed to complete treatment, left treatment prior to its end, or refused to return to treat‐
ment” (n = 53, 69.7%). Finally, efficacy-type studies (n = 42, 55.3%) were more than effec‐
tiveness-type studies (n = 34, 44.7%).

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01

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; U

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te
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ua

l
D

is
ab

ili
ty

34
A

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10

.0
0

11
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1
38

77
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3

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ic

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r 

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 a

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(2

01
3)

; U
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22
2

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45

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5

46
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45

62
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3

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hr

m
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t a

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(2

01
3)

; E
ur

op
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P

76
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nl
in

e 
D

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cu

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ru
m

15
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9
15

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9

49
59

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7

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 e

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(2
01

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; U

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ys
ic

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54

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uc

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20

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; E
ur

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t r
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ta

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61

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43
67

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01

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; E

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 C
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(2
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; U

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87
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20

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9

36
.3

6
28

46
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(2
01

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; A

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 F

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; E
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3
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9

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79
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 a

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; U
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45
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; E
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bs

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2

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I
7.

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12

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0

46
78

O
In

d
10

Karekla, Konstantinou, Ioannou et al. 11

Clinical Psychology in Europe
2019, Vol.1(3), Article e33058
https://doi.org/10.32872/cpe.v1i3.33058

https://www.psychopen.eu/


St
ud

y 
&

 R
eg

io
n

D
is

or
de

r
N

C
on

tr
ol

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ro

up
(s

)
%

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ro

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C

T

%
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s)
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%
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tt
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ax

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 &
 B

on
d 

(2
01

0)
; E

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D

is
tr

es
s

10
7

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T

13
.5

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10

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1

39
72

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3
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(2

00
7)

; U
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nx

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10
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T

33
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0

28
80

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M

 =
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27

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(2
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; U

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12

8
SB

T
9.

46
20

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10

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40

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; U

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40
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; U

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13

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16

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(2
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; E

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46
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; U

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36
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; U
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33

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; U

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33

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In

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et
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;
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(2
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; U

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; U
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20

11
);

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 R

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; U

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81

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0
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9)

; E
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81
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37

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(2

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2)

; E
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99
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2.

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01

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; U

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U

15
.1

5
8.

11
27

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0

In
p

G
M

 =
 3

.9
1

Treatment Dropout in ACT 12

Clinical Psychology in Europe
2019, Vol.1(3), Article e33058
https://doi.org/10.32872/cpe.v1i3.33058

https://www.psychopen.eu/


St
ud

y 
&

 R
eg

io
n

D
is

or
de

r
N

C
on

tr
ol

 g
ro

up
(s

)
%

 d
ro

po
ut

A
C

T

%
 d

ro
po

ut
co

n
tr

ol
gr

ou
p(

s)
M

ea
n

 A
ge

%
 f

em
al

e
Se

tt
in

g
Fo

rm
at

T
x 

w
ee

k
s

Ke
m

an
i e

t a
l. 

(2
01

6)
; E

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op

e
Pa

in
60

A
R

0.
00

16
.6

6
40

73
O

G
12

K
in

gs
to

n 
(2

00
8)

; E
ur

op
e

Tr
ea

tm
en

t r
es

is
ta

nt
40

C
BT

15
.0

0
40

.0
0

44
60

O
G

16
Ko

co
vs

ki
, F

le
m

in
g,

 H
aw

le
y,

 H
ut

a,
 &

A
nt

on
y 

(2
01

3)
; U

S
SA

D
13

7
C

BT
30

.1
9

39
.6

2
35

54
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12

La
ng

 e
t a

l. 
(2

01
7)

; U
S

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16

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PC

T
33

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5

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.0

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34

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O

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a,

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; E
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10

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 (2

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; U
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28

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6

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6)

; U
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(2
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; E

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15

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9.

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11

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48
96

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Fl

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 (2

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; U
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13

3
TA

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10

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34
46

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; E

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C

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73

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U

18
.9

2
2.

78
58

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cM

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2)

; E
ur

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I

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5

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12

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19
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34

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of

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t &

 M
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5)

; A
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Ph

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al
 A

ct
iv

ity
76

PW
P

0.
00

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40

44
83

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S

12
M

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tr

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(2
01

7)
; U

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34

TA
U

11
.7

7
5.

88
34

21
O

In
d

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 S

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 &
 G

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m

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2)

; A
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41

TA
U

23
.8

1
30

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93

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8)

; E
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 C

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47

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uc

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io

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21

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4

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56

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Ι

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M

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ta

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av

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(2

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2)

; A
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26
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7
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36

10
0

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 (2

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21

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9.

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46

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7)

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73
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43

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0

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98

O
In

d
19

Karekla, Konstantinou, Ioannou et al. 13

Clinical Psychology in Europe
2019, Vol.1(3), Article e33058
https://doi.org/10.32872/cpe.v1i3.33058

https://www.psychopen.eu/


St
ud

y 
&

 R
eg

io
n

D
is

or
de

r
N

C
on

tr
ol

 g
ro

up
(s

)
%

 d
ro

po
ut

A
C

T

%
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ro
po

ut
co

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tr

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gr

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p(

s)
M

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n

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ge

%
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al

e
Se

tt
in

g
Fo

rm
at

T
x 

w
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k
s

Pe
te

rs
en

 &
 Z

et
tle

 (2
00

9)
; U

S
C

om
or

bi
d

de
pr

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on
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nd
al

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ho

l u
se

24
TA

U
20

.0
0

7.
69

38
50

In
p

In
d

M
 =

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ot

t e
t a

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8)

; E
ur

op
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P

63
SP

M
25

.8
1

21
.8

8
46

64
S

In
d

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s
44

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fr

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76

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09

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44

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In

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7)

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36

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uc

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5.

66
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67

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09

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79

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11
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he

re
ll 

et
 a

l. 
(2

01
1a

); 
U

S
A

nx
ie

ty
21

C
BT

0.
00

44
.4

4
71

48
O

In
d

12
W

et
he

re
ll 

et
 a

l. 
(2

01
1b

); 
U

S
C

P
11

4
C

BT
10

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3

14
.0

3
55

51
O

G
8

W
hi

te
 e

t a
l. 

(2
01

1)
; E

ur
op

e
Ps

yc
ho

si
s

27
TA

U
0.

00
23

.0
8

34
22

O
In

d
10

Treatment Dropout in ACT 14

Clinical Psychology in Europe
2019, Vol.1(3), Article e33058
https://doi.org/10.32872/cpe.v1i3.33058

https://www.psychopen.eu/


St
ud

y 
&

 R
eg

io
n

D
is

or
de

r
N

C
on

tr
ol

 g
ro

up
(s

)
%

 d
ro

po
ut

A
C

T

%
 d

ro
po

ut
co

n
tr

ol
gr

ou
p(

s)
M

ea
n

 A
ge

%
 f

em
al

e
Se

tt
in

g
Fo

rm
at

T
x 

w
ee

k
s

W
ic

ks
el

l, 
M

el
in

, L
ek

an
de

r, 
&

 O
ls

so
n

(2
00

9)
; E

ur
op

e
LP

P
32

M
D

T
0.

00
0.

00
15

78
O

In
d

12

W
ol

itz
ky

-T
ay

lo
r, 

A
rc

h,
 R

os
en

fie
ld

, &
C

ra
sk

e 
(2

01
2)

; U
S

A
nx

ie
ty

12
1

C
BT

36
.3

6
25

.7
6

38
57

O
In

d
12

Ze
tt

le
 (2

00
3)

; U
S

M
at

he
m

at
ic

s
A

nx
ie

ty
24

SD
14

.2
9

36
.8

4
31

83
O

In
d

6

N
ot

e.
 N

I =
 n

ot
 in

di
ca

te
d;

 U
S 

= 
U

ni
te

d 
St

at
es

; O
C

D
 =

 O
bs

es
si

ve
 C

om
pu

ls
iv

e 
D

is
or

de
r; 

SP
 =

 S
oc

ia
l P

ho
bi

a;
 M

U
D

 =
 M

et
ha

m
ph

et
am

in
e 

U
se

 D
is

or
de

rs
; S

A
D

 =
 S

oc
ia

l
A

nx
ie

ty
 D

is
or

de
r; 

G
A

D
 =

 G
en

er
al

iz
ed

 A
nx

ie
ty

 D
is

or
de

r; 
C

P 
= 

C
hr

on
ic

 P
ai

n;
 L

PP
 =

 L
on

gs
ta

nd
in

g 
pe

di
at

ri
c 

pa
in

; T
BI

 =
 T

ra
um

at
ic

 B
ra

in
 In

ju
ry

; B
S 

= 
Ba

ri
at

ri
c 

Su
r‐

ge
ry

; E
D

 =
 E

at
in

g 
di

so
rd

er
s;

 B
PD

 =
 B

or
de

rl
in

e 
Pe

rs
on

al
ity

 D
is

or
de

r; 
M

S 
= 

M
in

im
al

 S
up

po
rt

 G
ro

up
; C

BT
 =

 C
og

ni
tiv

e 
Be

ha
vi

or
al

 T
he

ra
py

; C
T 

= 
C

og
ni

tiv
e 

Th
er

ap
y;

PR
T 

= 
Pr

og
re

ss
iv

e 
Re

la
xa

tio
n 

Tr
ai

ni
ng

; S
D

 =
 S

ys
te

m
at

ic
 D

es
en

si
tiz

at
io

n;
 S

SR
Is

 =
 S

el
ec

tiv
e 

Se
ro

to
ni

n 
Re

up
ta

ke
 In

hi
bi

to
rs

; N
C

C
 =

 N
on

-s
ta

nd
ar

di
ze

d 
C

on
tr

ol
 C

on
di

‐
tio

n;
 R

PT
 =

 R
ec

om
m

en
de

d 
Ph

ar
m

ac
ol

og
ic

al
 T

re
at

m
en

t; 
TA

U
 =

 T
re

at
m

en
t a

s 
U

su
al

; M
M

 =
 M

et
ha

do
ne

 m
ai

nt
en

an
ce

; I
TS

F 
= 

In
te

ns
iv

e 
Tw

el
ve

 S
te

p 
Fa

ci
lit

at
io

n 
Th

er
a‐

py
 P

lu
s 

M
et

ha
do

ne
 M

ai
nt

en
an

ce
; P

E 
= 

Ph
ys

ic
al

 E
xe

rc
is

e;
 A

R 
= 

A
pp

lie
d 

Re
la

xa
tio

n;
 M

D
T 

= 
M

ul
tid

is
ci

pl
in

ar
y 

tr
ea

tm
en

t a
nd

 a
m

itr
ip

ty
lin

e;
 M

TA
U

 =
 m

ed
ic

al
 tr

ea
t‐

m
en

t a
s 

us
ua

l; 
EW

 =
 E

xp
re

ss
iv

e 
w

ri
tin

g;
 S

BT
 =

 S
ta

nd
ar

d 
Be

ha
vi

or
al

 T
re

at
m

en
t; 

SP
M

 =
 S

pe
ci

al
ty

 M
ed

ic
al

 M
an

ag
em

en
t; 

BP
 =

 B
up

ro
pi

on
 R

eg
im

en
; N

RT
 =

 N
ic

ot
in

e
Re

pl
ac

em
en

t T
re

at
m

en
t; 

A
BA

 =
 A

pp
lie

d 
Be

ha
vi

or
 A

na
ly

si
s;

 P
W

P 
= 

Pe
do

m
et

er
-b

as
ed

 w
al

ki
ng

 p
ro

gr
am

; T
RT

 =
 T

in
ni

tu
s 

Re
tr

ai
ni

ng
 T

he
ra

py
; W

D
I =

 W
or

kp
la

ce
 D

ia
‐

lo
gu

e 
In

te
rv

en
tio

n;
 R

T 
= 

Re
la

xa
tio

n 
Tr

ai
ni

ng
; B

A
 =

 B
eh

av
io

ra
l A

ct
iv

at
io

n;
 S

IT
 =

 S
tr

es
s 

In
oc

ul
at

io
n 

Tr
ai

ni
ng

; H
A

B 
= 

Ex
po

su
re

 w
ith

 H
ab

itu
at

io
n 

Ra
tio

na
le

; P
C

T 
=

Pr
es

en
t-

C
en

te
re

d 
Th

er
ap

y;
 tC

BT
 =

 T
ra

di
tio

na
l C

og
ni

tiv
e 

Be
ha

vi
or

al
 T

he
ra

py
; O

 =
 O

ut
pa

tie
nt

; I
np

 =
 In

pa
tie

nt
; S

 =
 S

el
f-

he
lp

; G
 =

 G
ro

up
; I

nd
 =

 In
di

vi
du

al
.

Karekla, Konstantinou, Ioannou et al. 15

Clinical Psychology in Europe
2019, Vol.1(3), Article e33058
https://doi.org/10.32872/cpe.v1i3.33058

https://www.psychopen.eu/


   Regarding reasons reported for dropout, the majority of studies did not report data 
about client variables separately for dropout and completers. Of the 65 studies presenting 
drop‐outs in the ACT condition, only 27 studies (41.54%) reported reasons for dropout. 
Regard‐ing comparisons, all participants completed treatment in 11 studies, whereas for 
the re‐maining 65 studies with dropouts, only 30 (45.15%) reported dropout reasons. For 
ACT, the most frequently reported reasons for dropout were: lost contact (n = 15, 55.55%), 
per‐sonal (n = 12, 44.44%), transportation difficulties (n = 10, 37.04%) and therapy factors 
(n = 9, 33.33%). However, for comparison condition(s) the main reasons for dropout were: 
lost contact (n = 19, 63.33%), therapy factors (n = 11, 36.67%) and time demands (n = 10, 
33.33%). For percentages of clients reporting each of the reasons for the included studies, 
see Appendix A in Supplementary Materials.

Dropout Rates
Across all studies and comparison conditions, the overall weighted mean dropout rate 
was 17.95%, 95% CI [15.12, 20.77]. ACT trials reported an average dropout rate of 17.35%, 
95% CI [14.33, 20.37] and comparison conditions reported an average dropout rate of 
18.62%, 95% CI [15.29, 21.96]. In the CMA, the unconstrained model with the 76 studies of 
ACT vs. comparison conditions, showed that the heterogeneity detected using the fixed-
effects model was very small and non-significant, with Q(75) = 79.371, p = .343, I2 = 
5.507%. It was thus justifiable to hypothesize that the random errors among the studies 
were not considerably different and that fixed-effects models could be followed. Despite 
dropout rates in ACT appearing to be lower than in comparison groups when examining 
the overall weighted mean dropout rate the difference did not reach statistical signifi‐
cance, as the point estimate of the odds ratio and its confidence intervals included value 1 
(i.e., equal odds/risk to dropout) with OR = 0.931, 95% CI [0.809, 1.070], z = -1.011,
p = .312; see Appendix B in Supplementary Materials). The funnel plot for the investiga‐
tion of publication bias in the meta-analysis (see Appendix B in Supplementary Materi‐
als) indicated no asymmetry, suggesting that there was no statistically significant publi‐
cation bias, with Egger test t(74) = 0.591, 95% CI [-0.617, 0.334], p = .556) or the Begg and 
Mazumdar Kendall’s tau, with τ = -0.079, p = .313.

Quantitative Synthesis of the Findings of the Reviewed Studies: 
Meta-Analysis
Participant Moderators
Eight participant variables were first examined as moderators of therapy dropout (see 
Table 3).

Treatment Dropout in ACT 16

Clinical Psychology in Europe
2019, Vol.1(3), Article e33058
https://doi.org/10.32872/cpe.v1i3.33058

https://www.psychopen.eu/


Ta
bl

e 
3

R
es

ul
ts

 F
ro

m
 t

he
 S

ub
gr

ou
p 

A
na

ly
si

s 
of

 P
ar

ti
ci

pa
nt

, S
tu

dy
 a

nd
 P

ro
vi

de
r 

C
at

eg
or

ic
al

 M
od

er
at

or
s 

on
 T

he
ra

py
 D

ro
po

ut

M
od

er
at

or
N

M
ea

n
 D

ro
po

ut
 R

at
e

95
%

 C
I

Z
p

Q
P

A
C

T
C

on
tr

ol
A

C
T

C
on

tr
ol

C
li

en
t 

di
so

rd
er

7.
10

1
0.

52
6

A
nx

ie
ty

 d
is

or
de

r 
or

 s
oc

ia
l p

ho
bi

a
14

19
.4

1
27

.2
7

13
.0

3,
 2

5.
79

21
.0

5,
 3

3.
49

-0
.6

63
0.

50
7

D
ep

re
ss

io
n

2
16

.8
2

19
.4

7
10

.5
7,

 2
3.

06
-3

.6
2,

 4
2.

56
-0

.5
67

0.
57

1
Su

bs
ta

nc
e 

ab
us

e 
or

 d
ep

en
de

nc
e

6
28

.7
3

24
.9

1
11

.0
8,

 4
6.

38
4.

72
, 4

5.
09

0.
12

2
0.

90
3

O
th

er
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ea
lth

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la

te
d 

pr
ob

le
m

s
10

12
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0
12

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0

5.
88

, 2
0.

01
7.

16
, 1

8.
64

0.
06

4
0.

94
9

C
hr

on
ic

 P
ai

n
14

18
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3
11

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3

10
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9,
 2

5.
66

5.
61

, 1
8.

24
0.

73
8

0.
46

1
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on

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ea
lth

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on

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tio

ns
7

13
.8

6
12

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3

6.
92

, 2
0.

81
6.

33
, 1

8.
13

0.
21

5
0.

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9

Ea
tin

g 
pa

th
ol

og
y/

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so

rd
er

8
12

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9

9.
83

6.
86

, 1
8.

32
4.

88
, 1

4.
79

0.
37

9
0.

70
5

Sm
ok

in
g

5
31

.1
0

32
.2

3
16

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5,

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5.

55
16

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1,

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7.

65
-1

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65

0.
20

6
Se

ve
re

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sy

ch
op

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ho

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10
10

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8

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84
, 1

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92

12
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 3

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47

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6

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re

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nt

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44
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du

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18

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 2

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1.

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68

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U

ni
te

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at
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32
19

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5

23
.0

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15

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 2
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 2
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-1

.5
38

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12

4
A

us
tr

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ia

8
17

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2

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4.

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, 2

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62

9.
60

, 3
3.

76
-0

.4
41

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65

9
Eu

ro
pe

29
16

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8

14
.8

8
11

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0,

 2
0.

86
10

.9
0,

 1
8.

86
0.

49
3

0.
62

2
A

si
a

7
10

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6

10
.3

3
3.

91
, 1

8.
01

2.
98

, 1
7.

68
0.

22
1

0.
82

5

T
h

er
ap

is
t 

ex
pe

ri
en

ce
 le

ve
l i

n
 A

C
T

 g
ro

up
s

5.
61

1
0.

46
8

M
as

te
rs

/ m
as

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r 

le
ve

l t
he

ra
pi

st
s 

or
 d

oc
to

ra
l

st
ud

en
ts

, i
nt

er
ns

, r
es

id
en

ts
13

23
.2

6
24

.2
0

14
.5

6,
 3

1.
96

15
.5

3,
 3

2.
87

0.
74

7
0.

45
5

Ph
D

 th
er

ap
is

ts
, d

oc
to

ra
te

15
15

.2
4

14
.2

5
8.

69
, 2

1.
80

9.
01

, 1
9.

50
0.

00
6

0.
99

5
M

ix
 P

hD
, s

tu
de

nt
s 

an
d 

ot
he

rs
15

16
.3

4
21

.9
0

10
.1

7,
 2

2.
52

13
.3

2,
 3

0.
48

-2
.3

66
0.

01
8

N
ot

 in
fo

rm
ed

5
10

.9
0

11
.5

7
3.

08
, 1

8.
73

0.
20

, 2
2.

94
-0

.7
38

0.
46

1
Ps

yc
ho

lo
gi

st
s 

m
ix

ed
 le

ve
ls

19
16

.1
5

18
.3

5
10

.3
4,

 2
1.

95
11

.4
3,

 2
5.

26
-0

.0
80

0.
93

6
N

o 
th

er
ap

is
ts

 (o
nl

in
e)

8
19

.9
2

17
.4

3
8.

87
, 3

1.
16

6.
10

, 2
8.

75
0.

05
9

0.
95

3
N

on
-p

sy
ch

ol
og

is
ts

1
21

.7
4

12
.5

0
-

-
0.

83
3

0.
40

5

Karekla, Konstantinou, Ioannou et al. 17

Clinical Psychology in Europe
2019, Vol.1(3), Article e33058
https://doi.org/10.32872/cpe.v1i3.33058

https://www.psychopen.eu/


M
od

er
at

or
N

M
ea

n
 D

ro
po

ut
 R

at
e

95
%

 C
I

Z
p

Q
P

A
C

T
C

on
tr

ol
A

C
T

C
on

tr
ol

T
h

er
ap

is
t 

ex
pe

ri
en

ce
 le

ve
l i

n
 c

om
pa

ri
so

n
 g

ro
up

s
5.

99
0

0.
54

1
M

as
te

rs
/ m

as
te

r 
le

ve
l t

he
ra

pi
st

s 
or

 d
oc

to
ra

l
st

ud
en

ts
, i

nt
er

ns
, r

es
id

en
ts

10
24

.3
7

23
.1

0
14

.6
5,

 3
4.

09
13

.3
8,

 3
2.

82
0.

76
5

0.
44

4

M
ix

 P
hD

, s
tu

de
nt

s 
an

d 
ot

he
rs

15
14

.9
9

20
.5

8
9.

82
, 2

0.
15

14
.2

1,
 2

6.
95

-1
.4

79
0.

13
9

Ps
yc

hi
at

ri
st

s
2

34
.7

4
42

.4
6

31
.4

9,
 3

7.
99

33
.9

1,
 5

1.
01

-2
.0

87
0.

03
7

Ph
D

 th
er

ap
is

ts
, d

oc
to

ra
te

9
17

.8
5

17
.8

4
7.

37
, 2

8.
34

10
.2

8,
 2

5.
41

1.
22

1
0.

22
2

N
on

-p
sy

ch
ol

og
is

ts
 (d

ru
g 

st
af

f, 
al

co
ho

l
co

un
se

lo
r, 

ph
ys

ic
ia

n,
 p

sy
ch

o-
ph

ar
m

ac
ol

og
is

t)
9

13
.4

9
18

.0
6

8.
05

, 1
8.

94
7.

18
, 2

8.
94

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lie

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od
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us

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ra
nd

om
-e

ffe
ct

s 
an

al
ys

is
.

k 
= 

nu
m

be
r 

of
 re

le
va

nt
 s

tu
di

es
 in

cl
ud

ed
 in

 e
ac

h 
an

al
ys

is
; M

ea
n 

D
ro

po
ut

 R
at

e 
= 

th
e 

m
ea

n 
pe

rc
en

ta
ge

 o
f p

ar
tic

ip
an

ts
 te

rm
in

at
in

g 
pr

em
at

ur
el

y;
 C

I =
 c

on
fid

en
ce

 in
te

r‐
va

ls
; Z

 =
 tw

o-
ta

ile
d 

te
st

 in
di

ca
tin

g 
w

hi
ch

 le
ve

ls
 o

f t
he

 m
od

er
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or
s 

ar
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gn

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ca

nt
; Q

 =
 te

st
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f h
et

er
og

en
ei

ty
 b

et
w

ee
n 

le
ve

ls
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f e
ac

h 
m

od
er

at
or

.
*p

 <
 .0

5.
 **

p 
< 

.0
1.

 **
*p

 <
 .0

01
.

Treatment Dropout in ACT 18

Clinical Psychology in Europe
2019, Vol.1(3), Article e33058
https://doi.org/10.32872/cpe.v1i3.33058

https://www.psychopen.eu/


   Regarding categorical moderators, there were no significant differences between sub‐
groups. This was expected as heterogeneity among the studies was very small and the 
studies were generally favoring ACT groups but this finding did not reach statistical sig‐
nificance. However, separate investigation of the effect sizes in each subgroup of studies 
showed that a significant finding was noted in the subgroup analysis for the type of dis‐
order under investigation (see Appendix C in Supplementary Materials); where in studies 
with a population with a severe psychopathology (i.e., borderline personality disorder, 
treatment resistant and psychosis) the dropouts were significantly lower in ACT groups 
compared to comparisons (OR = 0.473, z = -2.473, p = .01). In terms of the six participant 
continuous moderators, meta-regression analyses based on the odds ratio using a meth‐
od-of-moments estimation showed that none of them (gender, marriage, ethnicity, em‐
ployment and mean age) were independent predictors of the effect size.

Treatment Moderators
Eight treatment variables were tested as moderators of dropout rate (see Table 4 for cate‐
gorical variables). Subgroup analyses of treatment setting showed again non-significant 
between-group differences for all the variables examined. However, a statistically signifi‐
cant effect was noted in the subgroup analysis using the recruitment setting, as having 
recruited the population from a public outpatient clinic and/or community advertise‐
ments resulted in significantly lower odds of the population to drop out from ACT 
groups compared to comparison groups, OR = 0.652, z = -2.985, p = .003. No significant 
differences were found among the rest of the examined treatment moderators and no 
other significant effect sizes in specific subgroups were noticed.

Provider and Study Moderators
Only a small amount of studies reported therapist gender, age, and ethnicity, deeming it 
impossible to analyze them as moderators. The experience levels of the therapists in ACT 
and comparison groups showed non-significant differences. However, in the subgroup 
analysis of the ACT therapists’ experience level a significant effect size was found for the 
subgroup of therapists from mixed experience levels, including doctoral level, student 
trainees, and others (e.g., licensed clinicians, psychiatrists, social workers, psychiatric 
nurses). The odds on dropout from ACT groups were significantly lower than from com‐
parison groups when the ACT therapists consisted of a multi-level and multi-domain 
team, with OR = 0.734, z = -2.366, p = .018. Also, in the subgroup analysis of the compari‐
son groups’ therapists a significant effect favoring ACT groups was found in the sub‐
group of psychiatrists. When the comparison groups had psychiatrists as the main and 
only therapists, then participants had significantly higher odds to dropout, compared to 
ACT groups, with OR = 0.638, z = -2.087, p = .037. Regarding study moderators, subgroup 
analyses based on region and type of study, or when examining the predictive ability of 
the year of publication in meta-regression analyses showed no significant results.

Karekla, Konstantinou, Ioannou et al. 19

Clinical Psychology in Europe
2019, Vol.1(3), Article e33058
https://doi.org/10.32872/cpe.v1i3.33058

https://www.psychopen.eu/


Ta
bl

e 
4

R
es

ul
ts

 F
ro

m
 t

he
 S

ub
gr

ou
p 

A
na

ly
si

s 
of

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re

at
m

en
t 

C
at

eg
or

ic
al

 M
od

er
at

or
s 

on
 T

he
ra

py
 D

ro
po

ut

M
od

er
at

or
N

M
ea

n
 D

ro
po

ut
 R

at
e

95
%

 C
I

Z
p

Q
P

A
C

T
C

on
tr

ol
A

C
T

C
on

tr
ol

T
re

at
m

en
t 

fo
rm

at
 in

 A
C

T
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ro
up

s
0.

13
1

0.
98

8
G

ro
up

34
15

.9
2

15
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6
12

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2,

 1
9.

52
11

.0
6,

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9.

47
-0

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54

0.
87

8
In

di
vi

du
al

30
16

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7

20
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6
11

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9,

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1.

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15

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2,

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6.

80
-0

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18

0.
60

5
C

om
bi

ne
d

3
38

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4

36
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1
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6
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lf-
he

lp
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2

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2

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es

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ip

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on

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f 

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cr

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tm

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tt

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g

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5

O
ut

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tie

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lin
ic

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ffi

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te

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w

ith
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os
pi

ta
l o

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m

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oo

l
11

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, 2
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iv

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e 

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tp

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t h

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(s
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0,

 2
7.

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-0
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35
0.

97
2

Treatment Dropout in ACT 20

Clinical Psychology in Europe
2019, Vol.1(3), Article e33058
https://doi.org/10.32872/cpe.v1i3.33058

https://www.psychopen.eu/


M
od

er
at

or
N

M
ea

n
 D

ro
po

ut
 R

at
e

95
%

 C
I

Z
p

Q
P

A
C

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tr

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A

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T

C
on

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ol

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om

pa
ri

so
n

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ro

up
2.

84
5

0.
72

4
C

BT
17

18
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3
24

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5

11
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8,
 2

6.
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16
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9,
 3

1.
61

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0.

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1

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U

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ed

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ly

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om

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f C
BT

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ly

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8

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ot

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 T

ab
le

d 
ar

e 
w

ei
gh

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d 

m
ea

n 
dr

op
ou

t r
at

es
 o

f t
re

at
m

en
t m

od
er

at
or

s 
us

in
g 

ra
nd

om
-e

ffe
ct

s 
an

al
ys

es
.

k 
= 

nu
m

be
r 

of
 re

le
va

nt
 s

tu
di

es
 in

cl
ud

ed
 in

 e
ac

h 
an

al
ys

is
; M

ea
n 

D
ro

po
ut

 R
at

e 
= 

th
e 

m
ea

n 
pe

rc
en

ta
ge

 o
f p

ar
tic

ip
an

ts
 te

rm
in

at
in

g 
pr

em
at

ur
el

y;
 C

I =
 c

on
fid

en
ce

 in
te

r‐
va

ls
; Z

 =
 tw

o-
ta

ile
d 

te
st

 in
di

ca
tin

g 
w

hi
ch

 le
ve

ls
 o

f t
he

 m
od

er
at

or
s 

ar
e 

si
gn

ifi
ca

nt
; Q

 =
 te

st
 o

f h
et

er
og

en
ei

ty
 b

et
w

ee
n 

le
ve

ls
 o

f e
ac

h 
m

od
er

at
or

.
*p

 <
 .0

5.
 **

p 
< 

.0
1.

 **
*p

 <
 .0

01
.

Karekla, Konstantinou, Ioannou et al. 21

Clinical Psychology in Europe
2019, Vol.1(3), Article e33058
https://doi.org/10.32872/cpe.v1i3.33058

https://www.psychopen.eu/


Sensitivity Analyses
We performed sensitivity analyses based on decisions taken before, or based on the pre‐
vious findings of the meta-analysis. The exclusion of the three studies that consisted of
dissertations, showed that the main effect did not change significantly, with OR = 0.951,
95% CI [0.826, 1.094], z = -0.705, p = .481, even though heterogeneity was slightly re‐
duced, with Q(72)= 73.808, p = .419, I2 = 2.450. The next sensitivity analysis concerned the
exclusion of studies with very wide confidence intervals of the odds ratio and showed
again no change of the main effect. Later on, we investigated the main effect when ex‐
cluding recent papers (2016-2018), as the meta-regression analysis for the predictive abili‐
ty of the year of publication showed a trend to significance. This sensitivity analysis (see
Appendix D in Supplementary Materials) showed that the main effect became marginally
significant, with OR = 0.852, 95% CI [0.727, 0.998], z = -1.984, p = .047, even though heter‐
ogeneity was slightly increased but remained at small levels, with Q(54) = 60.961,
p = .240, I2 = 11.418. The finding of the sensitivity analysis concerning the year of publi‐
cation suggested that when considering research done before 2016, the dropouts from
ACT groups were significantly lower than from active comparison groups.

D i s c u s s i o n
Treatment dropout is an important parameter impacting treatment outcomes (Barrett et
al., 2008; Wierzbicki & Pekarik, 1993). Despite the acknowledgement of the importance of
considering dropout rates and how these influence treatment effectiveness conclusions,
this phenomenon has not been extensively examined. This paper aimed to investigate the
phenomenon of dropout in a relatively newly developed therapeutic approach, Accept‐
ance and Commitment Therapy. Compared to other cognitive behavioral approaches,
ACT presents with advances in improving client engagement to treatment, emphasizes
the therapeutic relationship, and provides meaning for any changes to be made during
treatment, postulated to be associated with more participant engagement. Indeed, change
in values has been found to precede changes in suffering (Gloster et al., 2017). As such,
we aimed to examine if those advances presented in ACT could overcome some of the
treatment acceptability criticisms presented with older generations of interventions,
which may have contributed to increased dropout rates from psychological treatments.
However, the overall dropout rate was not significantly different between ACT and com‐
parison groups in the present meta-analysis.

As noted by others, we found that there is no consensus regarding the definition em‐
ployed by investigators. We adopted a broad definition of treatment dropout, utilizing
what was reported by each study author and particularly considering dropouts to be the
percentage of cases of individuals who began treatment but did not complete it as inten‐
ded by its developer. Based on this definition, the yielded overall dropout rate across all

Treatment Dropout in ACT 22

Clinical Psychology in Europe
2019, Vol.1(3), Article e33058
https://doi.org/10.32872/cpe.v1i3.33058

https://www.psychopen.eu/


studies included in this meta-analysis was 17.95%, which is comparable to recent previ‐
ously meta-analytically reported rates (i.e., 19.70%; Swift & Greenberg, 2012). For ACT,
the calculated mean dropout rate was 17.35%. This is similar again to the rates reported
by Galloway-Williams, Martin, Clum, and Cooper (2013) and Ong et al. (2018) for ACT.
When including all possible reasons and comparing across all comparison conditions, the
dropout rate was not significantly different (18.62%) from ACT groups. However, the rea‐
son why individuals terminate their treatment prematurely needs to be considered in re‐
lation to dropout rates.

Unfortunately, the majority of examined studies did not include dropout reasons, lim‐
iting our ability to draw conclusions regarding the reasons for dropout. Despite the limi‐
ted number of studies presenting reasons for dropouts, some important differences be‐
tween ACT and other groups were identified. For example, most individuals who drop‐
ped out from ACT groups did so because of lost contact and for (unrelated to therapy)
personal reasons. Dropout reasons in comparison conditions however, included addition‐
ally therapy-related reasons (e.g., not satisfied with the treatment or feeling that the ther‐
apy was too time consuming). In particular, when ACT was compared to CBT, the most
frequent reason for dropping out of CBT was therapy factors (i.e., of the 5 studies who
reported reasons for dropout from CBT, all of them mentioned therapy factors). In con‐
trast, in ACT, the reasons of time demands, transportation, personal and therapy were
equally reported. This is in line with findings reported by Karekla (2004) who found dif‐
ferences in the timing of dropout in relation to the treatment components between tCBT
and ACT participants. The pattern of dropout in tCBT was linked to the initiation of ex‐
posure whereas the same pattern was not found for those in the ACT condition (where
individuals who dropped out did so for unrelated reasons to treatment and discontinued
at different time points and not before exposure was introduced). These findings lend
support to the idea that ACT may be a more acceptable treatment choice over previous
waves of tCBT, and may better prepare (e.g., via use of values) individuals to engage and
ultimately benefit from even the most difficult of treatment content (e.g., exposure to
feared stimuli; see also Gloster et al., 2014, 2015). In the future, researchers are advised to
examine and report upon the timing and reasons for dropout.

In this review, we found that ACTs’ premature termination rates were lower for deal‐
ing with certain types of psychopathology (severe psychopathology). This finding may
highlight the important addition of ACT skills for severe psychopathologic conditions;
however this needs to be further explored. Interestingly, participants’ age did not moder‐
ate dropout rates, suggesting that all age groups result in similar dropout rates. This is a
divergence from the Swift and Greenberg (2012) meta-analysis, where younger individu‐
als had higher dropout rates (Barrett et al., 2008).

The subgroup analyses based on the description of the treatment setting showed that
dropout rates from ACT groups were lower for studies in which the treatment was deliv‐
ered in a public outpatient clinic and population was recruited by community advertise‐

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ments. However, one should note that these studies were highly heterogeneous, includ‐
ing participants with anxiety disorders, eating disorders, substance abuse, other health
problems, chronic pain, health conditions/chronic illnesses, smoking, severe psychopa‐
thology (i.e., BPD, psychosis, treatment resistant), and depression. Additionally, in most
of these studies the comparison condition was not another psychologically active inter‐
vention (i.e., in 57% of them the comparison group was treatment as usual, medication
only and education). Due to the high heterogeneity of these studies, this finding should
be interpreted with caution and further examined in the future.

In terms of provider moderators, experience level of providers in ACT and compari‐
son groups were significantly related to dropout rates. Specifically, when treatment was
delivered by a multi-level and multi-domain team, ACT had lower dropout rates than
comparison conditions. This is a divergence from the studies of Ong et al. (2018) and
Swift and Greenberg (2012), who reported no significant results when treatment was de‐
livered by multidisciplinary teams. In particular, in the study of Ong et al. (2018), ACT
had higher dropout rates than comparison groups when treatment was administered by
master's-level clinicians/therapists whereas in the study of Swift and Greenberg (2012)
dropout rates were higher when the treatment was provided by trainees. Differences be‐
tween these studies may be a result of the definition used for therapist experience level,
therefore more research is needed in this domain to be able to conclusively make recom‐
mendations as to the level of experience or the consistency of the therapeutic team that
leads to higher effect sizes. For the guidance of future researchers examining dropouts in
treatments, a checklist of definitions and variables to be collected which can be utilized
before, during and while reporting their findings, to ensure that adequate information re‐
garding dropouts is available, is presented in Appendix E (see Supplementary Materials).

Limitations
This study has several limitations that need to be considered in the interpretation of find‐
ings. First, the inclusion criteria were made broad enough in order to include a large
number of studies. All age groups were included; as well as various psychopathological
and non-psychopathological problems, and studies combining ACT with other interven‐
tions or medication. Though we attempted to deal with this heterogeneity in the disor‐
ders, interventions, populations and age conditions by examining moderators of interest,
this heterogeneity may have still affected the clarity of any differences between ACT and
comparison groups on dropout rates.

A second limitation may be related to the coding procedure. Specifically, for the vari‐
able of comparison condition, when a study had two comparison conditions we selected
to compare only the active treatment (e.g., CBT) and excluded the inactive comparison
condition (e.g., wait-list). A third limitation has to do with reasons reported for dropouts.
Specifically, the majority of studies did not report dropout reasons, making conclusions
about true reasons for dropout impossible or biased for the studies that reported these

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reasons. In order to further elucidate the phenomenon of treatment dropout, future stud‐
ies should examine and report reasons why participants drop out as well as the timing
when this occurs. Finally, in our meta-analysis it was not possible to carry out a compari‐
son between the demographic characteristics of dropouts and completers due to insuffi‐
cient data provided by studies. We would like to encourage researchers to ensure that
they report information separately for completers and dropouts so as to facilitate further
understanding into the phenomenon of dropout.

Clinical Implications
This review examined dropout rates of a third wave CBT intervention in a range of disor‐
ders, populations, ages and comparison conditions. Our findings show that overall drop‐
out rates between ACT and comparison conditions were not found to differ significantly.
Additionally, moderation analyses suggest that experience level of therapists in ACT and
comparison conditions, description of treatment setting, and client diagnosis are associ‐
ated with an increased likelihood of dropout. Therefore, interventions aiming to lower
attrition should plan a-priori how to better engage users belonging to these groups.

Our findings suggest that ACT may present some potential advances for improving
client engagement and retention, such as emphasizing that any behavior change needs to
be linked with the persons’ values, or it may include more interesting treatment content
through the use of metaphors and experiential exercises. However, more research is still
needed prior to being able to assertively make these conclusions.

Future Research
The findings of the present study offer possible hypotheses about which therapeutic pro‐
cesses are associated with client retention. However, more studies are needed that will
examine particular reasons for premature treatment termination, timing when this phe‐
nomenon occurs and how it may be linked to specific treatment components, and associ‐
ated variables in third wave treatments.

Moderators of the dropout effect for different therapeutic approaches are critical in
that they illuminate areas that may still have potential for improvement in the context of
an otherwise effective intervention. This needs to be further examined. For example,
even if ACT has lower dropout rates than some comparison conditions overall, but fe‐
males drop out more from ACT than comparison conditions, then ACT may need to con‐
sider how females are being engaged in the intervention and attempt to find ways to im‐
prove engagement (e.g., maybe more gender sensitive metaphors). Additionally, common
vs. specific factors in the psychotherapies being examined and in relation to how these
may affect dropout also need to be examined. Researchers are encouraged to examine
and report the reasons for dropout when a person discontinues the treatment premature‐
ly. Further understanding of these reasons should allow us to examine whether it is dis‐

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satisfaction with the common factors (e.g., therapeutic alliance, expectations, cultural
adaptations, empathy) that contribute to premature discontinuation or whether dropout
is related to specific factors (e.g., specific ingredients of the intervention provided). It is
essential that participant engagement and premature termination continue to serve as
topics of exploration in the clinical psychology arena, so as to improve the effectiveness
of interventions, decrease treatment dropout rates, and enhance the possible treatment
effects for participants.

Funding: The authors received no financial support for the research, authorship, and/or publication of this article.

Competing Interests: The authors declare no conflicts of interest.

Acknowledgments: The authors have no support to report.

Data Availability: Datasets for the studies are freely available (see the Supplementary Materials section).

S u p p l e m e n t a r y  M a t e r i a l s
The following data and materials are available for this study (for access see Index of Supplementa‐
ry Materials below):

 
Via the PsychArchives repository:

• Appendix A: Percentages of clients reporting each of the reasons for the included studies
• Appendix B: Forest and funnel plots of included studies
• Appendix C: Forest plot of subgroup analyses based on the type of disorder under investigation
• Appendix D: Sensitivity analysis for the year of publication
• Appendix E: Checklist of definitions and variables to be collected in order to properly document

dropouts

Via the International Prospective Register of Systematic Reviews (PROSPERO):

• Preregistered protocol (CRD42017068456) of the current study

Index of Supplementary Materials

Karekla, M., Konstantinou, P., Ioannou, M., Kareklas, I., & Gloster, A. T. (2019). Supplementary
materials to "The phenomenon of treatment dropout, reasons and moderators in Acceptance and
Commitment Therapy and other active treatments: A meta-analytic review". PsychOpen.
https://doi.org/10.23668/psycharchives.2594

Karekla, M., Konstantinou, P., Ioannou, M., Kareklas, I., & Gloster, A. T. (2017, July 7). Treatment
drop-out in acceptance and commitment therapy compared to other treatments: A systematic

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review and meta-analysis. (PROSPERO 2017 CRD42017068456). PROSPERO.
https://www.crd.york.ac.uk/PROSPERO/index.php

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https://doi.org/10.1037/0735-7028.24.2.190
https://doi.org/10.1037/a0029418
https://doi.org/10.1007/BF03395440
https://www.psychopen.eu/

	Treatment Dropout in ACT
	(Introduction)
	Current Study

	Method
	Literature Search
	Eligibility Criteria
	Coding Procedures
	Data Analysis

	Results
	Characteristics of Reviewed Studies
	Dropout Rates
	Quantitative Synthesis of the Findings of the Reviewed Studies: Meta-Analysis
	Sensitivity Analyses

	Discussion
	Limitations
	Clinical Implications
	Future Research

	(Additional Information)
	Funding
	Competing Interests
	Acknowledgments
	Data Availability

	Supplementary Materials
	References