Response to the Commentary „Can a 1-Item Scale for Psychotherapy Outcomes Be Psychometrically Robust?” Letter to the Editor, Commentary Response to the Commentary „Can a 1-Item Scale for Psychotherapy Outcomes Be Psychometrically Robust?” Brian Schwartz 1 , Miguel M. Gonçalves 2 , Wolfgang Lutz 1 , João Tiago Oliveira 2 , Suoma E. Saarni 3,4,5 , Orya Tishby 6 , Michael Barkham 7 , European Psychotherapy Consortium (EPoC) [1] Department of Psychology, Trier University, Trier, Germany. [2] CIPsi – Psychology Research Center, School of Psychology, University of Minho, Braga, Portugal. [3] Department of Psychiatry, Faculty of Medicine and Health Technology, Tampere University, Tampere, Finland. [4] Department of Psychiatry, Helsinki University Hospital HUS and Helsinki University, Helsinki, Finland. [5] Department of Psychiatry, Wellbeing Services County of Päijät-Häme, Lahti, Finland. [6] Department of Psychology, Hebrew University, Jerusalem, Israel. [7] Clinical and Applied Psychology Unit, School of Psychology, University of Sheffield, Sheffield, United Kingdom. Clinical Psychology in Europe, 2025, Vol. 7(1), Article e16921, https://doi.org/10.32872/cpe.16921 Published (VoR): 2025-02-28 Corresponding Author: Brian Schwartz, Clinical Psychology and Psychotherapy, Department of Psychology, Trier University, D-54286 Trier, Germany. Phone: +49 651 201 4321. E-mail: schwartzb@uni-trier.de We thank the author(s) for their commentary in response to our article on the European Psychotherapy Consortium (EPoC) of the European Chapter of the Society for Psycho­ therapy Research (SPR) published in Clinical Psychology in Europe (Gonçalves et al., 2024) and welcome the opportunity to respond. Whilst we appreciate the detailed comments on our article made by the author(s), we would like to take this opportunity to primarily address the specific criticisms of the EPO-1 single-item scale. Overall, the commentary criticizes our article on a combined theoretical and psycho­ metric basis as a first, but so far, successful, attempt to coordinate the administration of patient outcome data collected during the course of treatment and implemented across Europe. Such criticism fails to recognize the practical impact of our strategic approach, which aims to significantly advance the paradigm of patient-focused research. Our objective has been to advance the field of psychological therapies and pave the way for the first steps towards coordinated data collection, data-based quality assurance, and practice-based evidence into the therapeutic process across national borders. The single-item Emotional and Psychological Outcome-1 measure (EPO-1) addresses the real­ ity of identifying a common measurement language across different clinics in different 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. https://crossmark.crossref.org/dialog/?doi=10.32872/cpe.16921&domain=pdf&date_stamp=2025-02-28 https://orcid.org/0000-0003-4695-4953 https://orcid.org/0000-0003-2575-7221 https://orcid.org/0000-0002-5141-3847 https://orcid.org/0000-0001-6624-8816 https://orcid.org/0000-0003-3555-9958 https://orcid.org/0000-0001-6321-8590 https://orcid.org/0000-0003-1687-6376 https://www.psychopen.eu/ https://cpe.psychopen.eu/ https://creativecommons.org/licenses/by/4.0/ https://creativecommons.org/licenses/by/4.0/ https://creativecommons.org/licenses/by/4.0/ European countries with varying structures of assessment and other healthcare systems, some with already existing measurement systems and some with none. Hence, we are attempting what we consider to be a unique program of research implementation that is both feasible in most clinical settings (i.e., minimal demands on patients, therapists, and healthcare systems) and yet has a grand vision in transcending national boundaries that are, so often, limitations to research collaboration. Against this background, the commentary fails to fully appreciate the objectives and value of the EPO-1 item’s intro­ duction. In addition to presenting our general view of the project, which is broader than that of the commentary’s author(s), we would also like to respond to some specific points of criticism below. The author(s) assumes that the EPO-1 item has limited reliability and validity. In response, we direct the author(s) to the high correlations of the instrument with other established outcome instruments, such as the Outcome Questionnaire-30 (OQ-30; r = .601), the Questionnaire for the Evaluation of Psychotherapy (FEP-2; r = .626), and the Patient Health Questionnaire-9 (PHQ-9; r = .630), which can be found in detail in Chapter 4 (Appendix) of Bergin and Garfield’s handbook of psychotherapy and behavior change (Lutz et al., 2021). The empirical data (n = 521) also show that the pre-post effect sizes, measured with the EPO-1, are as strongly related to the above instruments and that the individual effect sizes are comparable to those of the other measures. The EPO-1 pre-post-effect sizes refute the assumption that the single-item measure EPO-1 is less sensitive to change than multi-item scales (pre-post-effect sizes, e.g., EPO-1(Likert): d = 1.086; EPO-1(analogue): d = 1.469; BSI: d = 0.879; OQ-30: d = 1.320; Lutz et al., 2021). For future research, the author(s) recommends demonstrating the stability of the item over time without intervention, its change in response to intervention, and its correlations with established measures. The last two points (change sensitivity and convergent validity) have already been empirically demonstrated (see above; Lutz et al., 2021), leaving only the first point (reliability in the absence of intervention) to be addressed in future studies. Many points of criticism in the commentary refer to the general disadvantages of self-report questionnaires. They are not specifically related to single-item scales (e.g., the varying interpretation of questions by individual patients or the cognitive complexity of evaluating the item). Further, the author(s) suggests extending the EPO-1 item with an alliance item and using large language models (LLMs), which are important and topical issues in recent psychotherapy research. However, the EPO-1 is intentionally designed as a single-item measure to ensure easy implementation in clinical practice. Moreover, this outcome measure assesses psychological well-being, not therapeutic alliance. While LLMs have become valuable tools in psychotherapy research, the EPO-1 item is a low-burden meas­ ure for patients, collected via self-report. Therefore, it should not be replaced by video or text analyses, which could capture a different perspective. Response to the Commentary by Scott T. Meier 2 Clinical Psychology in Europe 2025, Vol. 7(1), Article e16921 https://doi.org/10.32872/cpe.16921 https://www.psychopen.eu/ Our response addresses the concerns expressed based on theoretical considerations with empirical evidence. Furthermore, we would argue that the benefits and potential of these efforts to introduce a standardized outcome measure across Europe outweigh the theoretical (and, as demonstrated, not necessarily valid) criticisms. We are aware of the common problems of single-item scales, which is why the EPoC does not only focus on the EPO-1 item but also on developing and implementing crosswalks to create a standard measure structure between different clinics across Europe. In summary, EPoC is a project that aims to evolve and create large, heterogeneous data sets from different countries that will facilitate practice-based evidence and data- informed psychological therapy. EPO-1 is currently translated into 13 languages, and two more are expected to be added soon. We hope that more institutions will join our initiative in the future and adopt the item in their assessments. Ultimately, whether it is taken up in the field is an empirical question. Funding: This work was supported by the German Research Foundation (DFG) under project numbers 493169211, 504507043, and 525286173 granted to Wolfgang Lutz and partially conducted at the Psychology Research Centre (PSI/ 01662), School of Psychology, University of Minho, and supported by the Portuguese Foundation for Science and Technology and the Portuguese Ministry of Science, Technology and Higher Education (UID/PSI/01662/2019), through national funds (PIDDAC). Acknowledgments: The authors have no additional (i.e., non-financial) support to report. Competing Interests: Michael Barkham declares that he is a co-developer of the CORE-OM and CORE-10. Social Media Accounts: Brian Schwartz: Bluesky: @schwartz-psyres.bsky.social References Gonçalves, M. M., Lutz, W., Schwartz, B., Oliveira, J. T., Saarni, S. E., Tishby, O., Rubel, J. A., Boehnke, J. R., Montesano, A., Paiva, D., Ceridono, D., Zech, E., Willemsen, J., Saarni, S. I., Kompan Erzar, K., Janeiro, L., Gelo, O. C. G., Errázuriz, P., Holas, P., … Barkham, M. (2024). Developing a European Psychotherapy Consortium (EPoC): Towards adopting a single-item self-report outcome measure across European countries. Clinical Psychology in Europe, 6(3), Article 13827. https://doi.org/10.32872/cpe.13827 Lutz, W., De Jong, K., Rubel, J. A., & Delgadillo, J. (2021). Measuring, predicting and tracking change in psychotherapy. In M. Barkham, W. Lutz, & L. G. Castonguay (Eds.), Bergin and Garfield’s handbook of psychotherapy and behavior change (7th ed., pp. 89–133). Wiley. Schwartz, Gonçalves, Lutz et al. 3 Clinical Psychology in Europe 2025, Vol. 7(1), Article e16921 https://doi.org/10.32872/cpe.16921 https://bsky.app/profile/schwartz-psyres.bsky.social https://doi.org/10.32872/cpe.13827 https://www.psychopen.eu/ Clinical Psychology in Europe (CPE) is the official journal of the European Association of Clinical Psychology and Psychological Treatment (EACLIPT). PsychOpen GOLD is a publishing service by Leibniz Institute for Psychology (ZPID), Germany. Response to the Commentary by Scott T. Meier 4 Clinical Psychology in Europe 2025, Vol. 7(1), Article e16921 https://doi.org/10.32872/cpe.16921 https://www.psychopen.eu/