EUROPEAN JOURNAL OF PURE AND APPLIED MATHEMATICS Vol. 15, No. 4, 2022, 2074-2085 ISSN 1307-5543 – ejpam.com Published by New York Business Global An Approach of Estimating the Value at Risk of Heavy-tailed Distribution using Copulas Korotimi Ouédraogo1,∗, Diakarya Barro2 1 LANIBIO, UJKZ, BP: 7023 Ouagadougou 03, Burkina Faso 2 Université Thomas SANKARA, Burkina Faso Abstract. The value at risk (VaR) plays a fundamental role in modeling risk in financial studies. We propose a approach in estimating the VaR for heavy-tailed distribution by taking into account the effects of certain covariates on the variable of interest. This method, involves estimating the extreme conditional quantiles by using the assciated copula. Morever, we use Bernstein copulas to estimate the intermediate conditional quantile in a non-parametric approach of the direct method. Then, the extreme conditional quantile is also estimated and we study the asymptotic properties of this new estimator. 2020 Mathematics Subject Classifications: 91G70, 60G65, 62G32, 62H05 Key Words and Phrases: Copulas, Value at risk, extreme values, conditional quantiles, heavy- tailed distribution 1. Introduction The Value at Risk (VaR) is one of the best known risk measures in many fields such as finance and insurance. This measure quantifies the maximum loss that a portfolio manager can incur during a certain time horizon at a given confidence level. For a given level of confidence, the corresponding VaR for a random variable Y with distribution function F0 is given by V aRY (α) = QY (α) = inf {y ∈ R : F0(y) ≥ α} . (1) However, in some domains such as finance or actuarial science, the usual methods of estimating the VaR can be affected by factors such as interest rates or inflation. In the same vein, in many applications, the tail quantiles of the variable of interest Y depend on some covariate X from whom the VaR depends on. But, in practice its estimation does not take into account the effect of covariates on the variable of interest. So, it turns out to be necessary in the estimation of certain risk measures such as VaR, to take into account the influence of covariates. ∗Corresponding author. DOI: https://doi.org/10.29020/nybg.ejpam.v15i4.4280 Email addresses: korotimioued@gmail.com. (K. Ouédraogol Wahid), dbarro2@gmail.com (D. Barro) https://www.ejpam.com 2074 © 2022 EJPAM All rights reserved. K. Ouédraogo, D. Barro / Eur. J. Pure Appl. Math, 15 (4) (2022), 2074-2085 2075 A VaR of level α being a quantile of order α, the estimation of a value at risk in the presence of covariates consists in the estimation of a conditional quantile. Let (Y,X) be a pair of random variables such that Y is the univariate response or interest variable, with marginal distribution function F0 and X the covariate is a random vector with marginal distribution function F ; the conditional distribution function will be noted FY (• | X). The VaR under the effects of covariates of level α is defined by CoV aRY (α) = QY (α | X = x) = inf {y : FY (y | x) ≥ α} , (2) where x is a prespecified covariate vector. Several methods of estimating extreme conditional quantiles based on extreme values theory (EVT) exist. More particularly, J. Beirlant et al.[1] used univariate extreme theory and quantile regression to estimate conditional quantiles. Based on the nearest neighbours method, Gardes and al. [2] have proposed a method for estimating extreme conditional quantiles. However, this method suffers from the lack of data in the local neighbourhood. Based also on EVT and quantile regression, Wang and al. [3] proposed an estimation method assuming the linear dependence model. Furthermore, in 2013, Wang and al.[4] proposed another estimation method by relaxing the linearity assumption while Noh et al. [5] focused on a semi-parametric method for estimating conditional quantiles based on quantile regression and copulas. In 2017, Nasri & Bouezmarni [6] proposed two parametric and semi-parametric estimation methods based on copulas. In this study, we consider a random vector X = (X1;X2; ...;Xm)T of dimension m with joint distribution function F and marginal distribution function F1; ...;Fm. According to Sklar’s theorem [7] we can find a copula C such that F (x1; ...;xm) = C(F1(x1); ...;Fm(xm)). (3) If the marginals distributions functions F1, ..., Fm are continuous, then the copula C is unique. Conversly, if C is a m-dimensional copula an F1, ..., Fm univariate distributions functions, then the function F defined by (3) is a distribution function with margins F1, ..., Fm and C(u1, ..., um) = F (F−1 1 (u1), ..., F −1 m (um)). (4) More generaly, the copula C and the marginal distribution functions are not known and are estimated. Several methods of estimating copulas have been proposed. Parametric methods estimate the copula and the marginal distribution functions in a parametric way such as in Oakes [13], Romano and Joe [17]. Semi-parametric methods assume instead a parametric model for the copula and a non-parametric model for the marginal functions, see [10] , [11] , [14] and [23] . Finally there are the non-parametric estimation methods. It assumes a non-parametric model for the copulas and the marginal distributions. We can cite the work of Deuhevels [9], Gijbels and Mielniczuk [12]. In this work we use a non-parametric copula estimation method based on Bernstein polynomials to estimate the intermediate conditional quantile. Sancetta and Satchell [18] proposed in 2004 a method for estimating the copula function based on Bernstein K. Ouédraogo, D. Barro / Eur. J. Pure Appl. Math, 15 (4) (2022), 2074-2085 2076 polynomials called Bernstein copula. In the following, we consider the univariate case, i.e. m = 1, and we will use a non-parametric estimation of copulas for the estimation of our intermediate quantile. Our method of estimating extreme conditional quantiles is based on univariate EVT and the relationship between copulas and marginal distribution functions. The method consists in to use the copulas to estimate the intermediate conditional quantile and then using the EVT to estimate the extreme conditional quantile. EVT provides a elegant mathematicals tool for analyzing rares events. Our method of estimating the extreme conditional quantile differs from existing methods in that by using copulas namely Bern- stein copulas, the dependence relationship between the variable of interest and the covari- ate is general. In particular, Bernstein copulas provide a more adequate estimate of the underlying dependence structure. The paper is structured as follows. In the second section, we will discuss the various preliminaries that concern extreme values theory and copulas. In the section 3 we present our estimation method and study the asymptotic behaviour of our estimator. The last section consists of the conclusion followed by a discussion. 2. Materials and methods In the following sections, we estimate the intermediate conditional quantile based on a copulas and we introduce the notions of copulas with a focus on Bernstein ones. 2.1. An overview of extreme conditional quantiles. We suppose in the remainder of this work that FY (• | x) belongs to the domain of attraction of a distribution of extreme values Gγ (γ ∈ R). Let Z1; ...;Zn be a sample of random variables with distribution F and let Mn = max1≤i≤n Zi. The law of Mn suitably normalised converges to Gγ(x) = exp { −(1 + γz) −1 γ } ; (5) when n → ∞ and for 1 + γz > 0 According to the Fisher-Tippet-Gnedenko theorem (see [8]), Gγ belongs to one of the three following types of distributions: Gγ(x) =  exp [ −(x) −1 γ ] for x ≥ 0 and γ > 0 (Fréchet model) exp [− exp(−x)] for x ∈ R and γ = 0 (Gumbel model) exp [ −(−x) −1 γ ] for x < 0 (Weibull Negative model) . We are particularly interested in the case where γ > 0 i.e. the conditional distribution function FY (• | x) belongs to the Fréchet attraction domain. In this case the extreme K. Ouédraogo, D. Barro / Eur. J. Pure Appl. Math, 15 (4) (2022), 2074-2085 2077 conditional quantile is defined by: qY (αn | X = x) = ( 1− βn 1− αn )γ(x) qY (βn | x) ; (6) with βn such that βn → 1 and n(1− βn) → 0 when n → ∞ and the index of extreme values γ depends on the covariate X. 2.2. On the concept of Bernstein copulas The use of Bernstein Copulas in our estimation have many reasons.: • Firstly, it can approximate any behavior in the tail, and it recently has attracted attention in insurance modeling and is thus a natural candidate for further analysis. The Bernstein copulas are therefore relevant for the estimation of the CoVaR. • Secondly, the Bernstein copula is attractive from a modeling perspective. • Thirdly, the Bernstein copula is also suitable in higher dimensions, which is a major advantage compared to other parametric and non-parametric estimators. • Fourthly, its mathematical properties are interesting as the Bernstein estimator con- verges to the underlying dependence structure, provides a higher rate of consistency than other common nonparametric estimators [22]. In what follows, we first deal with some important result on copulas, the notion of Bernstein copulas and the relationship between the conditional distribution function and copulas. Consider (Y,X) a pair of random variables with marginal distribution functions F0 and F and joint distribution function H. According to relation (3) if F0 and F are continuous, then C is unique and we have C(u, v) = H(F−1 0 (u), F−1(v)). Given a sample of random variables (Y1, X1), ..., (Yn, Xn), the empirical estimator of the copula C is defined by, for all u, v ∈ [0, 1] by Cn(u, v) = Hn(F −1 0n (u), F−1 n (v)) with Hn(x, y) = n−1 n∑ i=1 I(Xi ≤ x, Yi ≤ y); Fn(x) = Hn(x,∞) = n−1 n∑ i=1 I(Xi ≤ x); and F0n(y) = Hn(∞, y) = n−1 n∑ i=1 I(Yi ≤ y). K. Ouédraogo, D. Barro / Eur. J. Pure Appl. Math, 15 (4) (2022), 2074-2085 2078 While modeling insurance tools, Sancetta & Satchelle [18] proposed the Bernstein copula function defined below as an estimator of the copula C Bp(u, v) = p∑ k=0 p∑ l=0 C ( k p , l p ) B(p, k, u)B(p, l, v) (7) where B(p, k, u) = Ck pu k(1−u)p−k is a binomial probability called Beirnstein’s polynomial. Note that limp→∞Bp(u, v) = C(u, v) is uniform over [0, 1]2 since C is continuous over [0, 1]2. However, this estimator depends on the copula C which is always unknown. They thus proposed in the above estimator to replace the copula C by its empirical estimator Cn [18]. This estimator is given by Cp,n(u, v) = p∑ k=0 p∑ l=0 Cn ( k p , l p ) B(p, k, u)B(p, l, v). (8) In the same vein, in the following paper [19] it has been showed that the almost certain convergence and the asymptotic normality of this estimator, see [22] and [23]. 2.2.1. Copulas and conditional distribution function The conditional quantile estimation based on copulas is based on quantile regression and the asymptotic distribution of this estimator has been proven to be Gaussian (see Noh et al [5]). Authors such as Kraus and Czado [20], Nasri and Bouezmarni [21] have used copulas based on the plug-in method. More recently, Remillard, Nasri and Bouezmarni [6] have used copulas and the direct estimation method of conditional quantiles to estimate the conditional quantile. The asymptotic normality of their estimator has been studied. We consider Y the one-dimensional random variable corresponding to the response variable of marginal distribution F0 and X = (X1, X2; ...;Xm)T a random vector of dimension m and of marginal distribution function F (x) = (F1(x1); ...;Fm(xm)). We note the conditional distribution of Y | X by FY (y | x) = P(Y ≤ y | X = x), y ∈ R, x = (x1, ..., xm) ∈ Rm. (9) We can establish a relation between the conditional distribution function, the copula function and the marginal distributions. This relation was established in its first version, i.e. in the case m = 1 in Bouyé & Salomon 14. In the case of dimension m ≥ 2 [6], the relation is given for all real x and y by: FY (y | x) = C̃(F0(y), F (x)); (10) where C̃ is the copula such that [16] C̃(F0(y), F (x)) = ∂F1(x1)...∂Fm(xm)C(F0(y), F1(x1), ..., Fm(xm)) ∂x1...∂xmC(1, F1(x1), ..., Fm(xm)) . (11) K. Ouédraogo, D. Barro / Eur. J. Pure Appl. Math, 15 (4) (2022), 2074-2085 2079 Now by definition of the copula C(1, F (x)) = F (x). So, equation(11) becomes C̃(F0(y), F (x)) = ∂F1(x1)...∂Fm(xm)C(F0(y), F1(x1), ..., Fm(xm)) f(x1, ..., xm) (12) In the rest of our work, we consider that m = 1, i.e. that X is a one-dimensional random variable. We therefore consider the pair of random variables (Y,X) of marginal distribution resp F0 and F and of conditional distribution FY (• | X). The relation between the copulas and the conditional distribution is now established as follows: FY (y | x) = C̃(F0(y), F (x)) = ∂C(F0(y), F (x)) ∂F (x) . (13) The conditional quantile function can be defined as the inverse of the conditional distri- bution function. For a conditional quantile of order α we have QY (α | x) = F−1 0 (Γ(α, F (x))) (14) where Γ(α, v) is the quantile of order α of the distribution function C̃(u, v) that is to say Γ(α, v) = inf {u ∈ [0, 1] | Γ(u, v) = α} and F−1 0 the generalized inverse of F0. In general, neither the copula C̃, nor the marginal distribution functions F0 and F are known. They must therefore be estimated. Using this method of estimating the conditional quantile we will estimate the intermediate quantile. Concretely, we will use the Bernstein copula estimator to estimate the copula C̃ and the empirical estimate for the marginal distribution functions. 3. Mains Results 3.1. Intermediate conditional quantile estimating by copulas Let αn be the order of the quantile such that αn → 1 when n → ∞. The conditional quantile is said to be intermediate if n(1−αn) → ∞ and extreme if n(1−αn) → K where K is a constant. In this work, we estimate an extreme VaR under the effects of covariates and therefore an extreme conditional quantile. Extreme value theory provides an ideal framework for these types of estimates. One considers a sequence (βn) defined such that βn → 1 and n(1−βn) → ∞ when n → ∞, the conditional quantile of order βn is an intermediate quantile noted QY (βn | X = x). We will thus estimate this intermediate conditional quantile by using the relation estab- lished between the copula functions, the marginal distribution functions and the condi- tional quantile functions. According to the equation (14), QY (βn | X = x) = F−1 0 (Γ(βn, F (x)) with Γ is the quantile of order βn of the distribution function C̃ , and F−1 0 is the generalized inverse of F0. We will first estimate the copula C̃ which corresponds to the conditional K. Ouédraogo, D. Barro / Eur. J. Pure Appl. Math, 15 (4) (2022), 2074-2085 2080 distribution and then we will estimate its quantile and finish by estimating the generalized inverse of the partial distribution F0. We first estimate the copula of the conditional distribution. By definition, we have: FY (y | x) = C̃(F0(y), F (x)) = ∂C(F0(y), F (x)) ∂F (x) . Like the copula C and the marginal distribution functions are generally not known, we will estimate them by using respectively Bernstein’s copulas for the copula C and empirical distribution functions for the marginal distribution functions. We call F0n and Fn the empirical distribution functions corresponding to the marginal distributions F0 and F The estimator of C using Bernstein’s copulas is given by Cp,n(F0n(y), Fn(x)) = p∑ k=0 p∑ l=0 Cn ( k p , l p ) B(p, k, F0n(y))B(p, l, Fn(x)) (15) where Cn is an empirical estimator of the copula C and B(p, k, F0n(y)) (resp) B(p, l, Fn(x) the Bernstein polynomials of order p in F0n(y) (resp) F0n(x)(x). We have B(p, l, Fn(x)) = C l p(Fn(x)) l(1− Fn(x)) p−l . By partially derivating Cp,n with respect to Fn(x), we obtain the copula C̃p,n given by the following result. Proposition 1. The estimator of the copule of conditional distribution is given by C̃p,n(F0n(y), Fn(x)) = A× Cp,n(F0n(y), Fn(x)). (16) with A defined by A = −p2Fn(x) + 5pFn(x)− 2p 2Fn(x)(1− Fn(x)) . If p = p(n) → ∞ and n p log logn → k ≥ 0 then ∥ C̃p,n −AC ∥=| A | ×O(n− 1 2 (log log n) 1 2 ) a.s n → ∞ (17) avec ∥ . ∥ the supremum norm The following proof is for the proposition (1) Proof. ∥ C̃p,n − AC ∥=| A |∥ Cp,n − C ∥. According to the theorem 1 of [19] ∥ Cp,n − C ∥= O(n− 1 2 (log log n) 1 2 . We then deduce the result 3.2. Estimation of the partial inverse of the copula C̃ This consists in finding the partial inverse of the copula C̃(F0(y), F (x)). It is defined by Γ(β(n), F (x)) = inf{F0(y) | C̃(F0(y), F (x)) = βn}. The intermediate conditional quantile is given by QY (βn | X = x) = F−1 0 (Γ(β(n), F (x))); (18) where F−1 0 is the generalized inverse of F0. K. Ouédraogo, D. Barro / Eur. J. Pure Appl. Math, 15 (4) (2022), 2074-2085 2081 Proposition 2. Let βn be the parameter of the intermediate conditional quantile. We fix βn = 1− k n such that k = k(n) → ∞ when n → ∞. The non-parametric estimator of the intermediate conditional quantile of order βn is defined by Q̂Y (βn | X = x) = F−1 0n { C−1 p,n ( −βnA −1, Fn(x) )} . (19) Proof. Recall that for a Bernstein polynomial B(m, k, z) it comes that: d dz B(m, k, z) = m [B(m− 1, k − 1, z)−B(m− 1, kz)] (20) and that Cp,n(F0(y), F (x)) = F0(y)F (x) p∑ k=0 p∑ l=0 Cn ( k p , l p ) B(p−1, k−1, F0(y))B(p−1, l−1, F (x)). (21) By definition we have C̃p,n(F0(y), F (x)) = p∑ k=0 p∑ l=0 Cn ( k p , l p ) B(p, k, F0(y)) d dF (x) B(p, l, F (x)). (22) Moreover from (20) we have equation (22) wich becomes: C̃p,n(F0(y), F (x)) = p∑ k=0 p∑ l=0 C ( k p , l p ) B(p, k, F0(y))p [B(p− 1, l − 1, F (x))−B(p− 1, l, F (x))] (23) Expanding and simplifying (23), we find C̃p,n(F0(y), F (x)) = B −D; (24) where B = p ( 1− F (x) 1− F (x) ) p∑ k=0 p∑ l=0 Cn ( k p , l p ) B(p, k, F0(y))B(p−1, l−1, F (x))−B(p−1, l, F (x)); and D = F (x) 1− F (x) p∑ k=0 p∑ l=0 lCn ( k p , l p ) B(p, k, F0(y))B(p− 1, l − 1, F (x)). So, since we have p∑ k=0 p∑ l=0 Cn ( k p , l p ) B(p, k, F0(y))B(p− 1, l − 1, F (x)) = 1 F (x) Cp,n(F0(y), F (x)); Then, it comes that B = p ( 1− F (x) 1− F (x) ) 1 F (x) Cp,n(F0(y), F (x)) K. Ouédraogo, D. Barro / Eur. J. Pure Appl. Math, 15 (4) (2022), 2074-2085 2082 and D = p(p+ 1) 2(1− F (x)) Cp,n(F0(y), F (x)). So, we obtain C̃p,n(F0(y), F (x)) = B +D = ACp,n(F0(y), F (x)). (25) Hence the value of A obtained in the previous definition. Using the formula Γ(β(n), F (x)) = inf{F0(y) | C̃(F0(y), F (x)) = βn} we obtain F0(y) = C−1 p,n(βnA −1, F (x)). (26) Moreoer by inversion of F0 we obtain: y = F−1 0 (C−1 p,n(βnA −1, F (x))) (27) Hence the value of the conditional quantile intermediate Let us make the following assumption. Hypothesis 1. We assume that the index of the extreme values γ of the conditional quantile is a constant and consequently does not depend on the covariate X Proposition 3. Consider a sample of n random variables for the response variable (Y1, X1), (Y2, X2), ..., (Yn, Xn). Let αn be defined such that αn → 1 and n(1−αn) → K where K is a constant. The non- parametric estimator of the extreme conditional quantile of order αn is defined by Q̂Y (αn | X = x) = ( 1− αn 1− βn )γF0 F−1 0 { C−1 p,n ( −βnA −1, Fn(x) )} (28) where γF0 is Hill estimator define by γF0 = 1 k ∑n k=0 log(Yn−i,n)− log(Yn−k,n). 3.3. Asymptotic results In this section, we establish the limiting behaviour of our different estimators. Proposition 4. Assuming the conditions of Theorem 2 in [19] are satisfied • If √ n p → 0 then for all (u, v) ∈ [0; 1]2 √ nA(C̃p,n(u, v)− C(u, v)) −→D N(0, A2σ2(u, v)). • If √ n p → d avec 0 < d < ∞ alors pour tout (u, v) ∈ [0; 1]2 √ nA(C̃p,n(u, v)− C(u, v)) −→D N(db(u, v), A2σ2(u, v)); REFERENCES 2083 with σ2(u, v) = C(u, v)(1− C(u, v)) + u(1− u)C2 u(u, v) + v(1− v)C2 v (u, v)(29) -2(1− u)C(u, v)Cu(u, v) −2(1− v)C(u, v)Cv(u, v) +2Cu(u, v)Cv(u, v) [C(u, v)− uv] and b(u, v) = 1 2 [u(1− u)Cuu(u, v) + v(1− v)Cvv(u, v)] where Cuu = ∂2 ∂u2 C(u, v) and Cu = ∂ ∂u C(u, v) Proof. According to the theorem 2 of [19], we have • If √ n p → 0 then for all (u, v) ∈ [0; 1]2 √ n(Cp,n(u, v)− C(u, v)) −→D N(0, σ2(u, v)) we deduce the precedent result • If √ n p → d avec 0 < d < ∞ alors pour tout (u, v) ∈ [0; 1]2 √ n(Cp,n(u, v)− C(u, v)) −→D N(db(u, v), σ2(u, v)), we also deduce the second result 4. 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