Efficient Concentration Risk Measurement in Credit Portfolios with Haar Wavelets

Transcription

1 Efficient Concentration Risk Measurement in Credit Portfolios with Haar Wavelets Josep J. Masdemont 1 and Luis Ortiz-Gracia 2 1 Universitat Politècnica de Catalunya 2 Centre de Recerca Matemàtica & Centrum voor Wiskunde en Informatica Jornada CRM-Empresa sobre Finanzas Cuantitativas Barcelona, February 22, 2013

2 Outline 1 Portfolio Credit Risk Modeling

3 Outline Introduction General Model Settings The Merton Model The ASRF Model and Concentration Risk 1 Portfolio Credit Risk Modeling Luis Ortiz-Gracia (CRM and CWI) CRM-Empresa 2013 Concentration Risk Measurement with Haar Wavelets 3 / 38

4 Introduction Introduction General Model Settings The Merton Model The ASRF Model and Concentration Risk It is very important for a bank to manage the risks originated from its business activities. The credit risk underlying the credit portfolio is often the largest risk in a bank. Basel Accords (I, II and III) laid the basis for international minimum capital standards. Banks became subject to regulatory capital requirements. Basel II is structured in a three Pillar framework: Pillar 1: more risk sensitive minimal capital requirements. Pillar 2: banks are allowed to calculate the economic capital (risk concentration). Pillar 3: transparency in bank s financial reporting. Luis Ortiz-Gracia (CRM and CWI) CRM-Empresa 2013 Concentration Risk Measurement with Haar Wavelets 4 / 38

5 Introduction Introduction General Model Settings The Merton Model The ASRF Model and Concentration Risk Concentration risks arise from an unequal distribution of loans to single borrowers (exposure or name concentration) or different industry or regional sectors (sector concentration). Merton model: basis of the Basel II IRB approach. Under homogeneity conditions, this model leads to the ASRF model. However, this model can underestimate risks in the presence of exposure concentration. Credit risk managers are interested in: How can concentration risk be quantified? How can risk measures be accurately computed in short times? Luis Ortiz-Gracia (CRM and CWI) CRM-Empresa 2013 Concentration Risk Measurement with Haar Wavelets 5 / 38

6 Risk Parameters Introduction General Model Settings The Merton Model The ASRF Model and Concentration Risk We specify a probability space (Ω, F, P) with filtration (F t ) t 0 satisfying the usual conditions. We fix a time horizon T > 0 (usually one year). We consider a credit portfolio consisting of N obligors. Any obligor n is characterized by: The exposure at default E n: potential exposure measured in currency. The loss given default L n: magnitude of likely loss on the exposure as a percentage of the exposure. The probability of default P n: likelihood that a loan will not be repaid. Each of them can be estimated from empirical default data. Luis Ortiz-Gracia (CRM and CWI) CRM-Empresa 2013 Concentration Risk Measurement with Haar Wavelets 6 / 38

7 Risk Measures Introduction General Model Settings The Merton Model The ASRF Model and Concentration Risk Consider an obligor n subject to default in the fixed time horizon T. We introduce D n, the default indicator of obligor n, { 1, if obligor n is in default, D n = 0, if obligor n is not in default, where P(D n = 1) = P n and P(D n = 0) = 1 P n. Let L be the portfolio loss given by, where L n = E n L n D n. L = N L n, n=1 Luis Ortiz-Gracia (CRM and CWI) CRM-Empresa 2013 Concentration Risk Measurement with Haar Wavelets 7 / 38

8 Risk Measures Introduction General Model Settings The Merton Model The ASRF Model and Concentration Risk Credit risk can split in Expected Losses EL (which can be forecasted) and Unexpected Losses UL (more difficult to quantify). Assumption 1.1 The exposure at default E n, the loss given default L n and the default indicator D n of an obligor n are independent. Denote by EL n the expectation value of L n, therefore, EL = E(L) = N E n EL n P n. n=1 Holding the UL = V(L) as a risk capital for cases of financial distress might not be appropriate (peak losses can be very large when they occur). Luis Ortiz-Gracia (CRM and CWI) CRM-Empresa 2013 Concentration Risk Measurement with Haar Wavelets 8 / 38

9 Risk Measures Introduction General Model Settings The Merton Model The ASRF Model and Concentration Risk Let α (0, 1) be a given confidence level, the α-quantile of the loss distribution of L in this context is called Value at Risk (VaR). Thus, VaR α = inf{l R : P(L l) α} = inf{l R : F L (l) α}, where F L is the cumulative distribution function of the loss variable L. VaR is the measure chosen in the Basel II Accord (α = 0.999) for the computation of capital requirement. Another important risk measure is the so called economic capital EC α for a given confidence level α, EC α = VaR α EL. Luis Ortiz-Gracia (CRM and CWI) CRM-Empresa 2013 Concentration Risk Measurement with Haar Wavelets 9 / 38

10 The One-Factor Merton Model Introduction General Model Settings The Merton Model The ASRF Model and Concentration Risk Let us consider the asset returns r n, r n = ρ n Y + 1 ρ n ɛ n, where Y and ɛ n, n are independent and standard normally distributed. Y usually denotes the business cycle and ɛ n the idiosyncratic shock. ρn represents the borrower n s sensitivity to systematic risk Y. D n = χ {rn<t n} B(1, P (r n < t n )), We have P n = P(r n < t n ), t n = Φ 1 (P n ) and, P n (y) P(r n < t n Y = y) = Φ ( tn ρ n y 1 ρn ), cond. default probability. Luis Ortiz-Gracia (CRM and CWI) CRM-Empresa 2013 Concentration Risk Measurement with Haar Wavelets 10 / 38

11 Portfolio Loss Introduction General Model Settings The Merton Model The ASRF Model and Concentration Risk Purpose: find an expression for the portfolio loss variable L. Assuming a constant loss given default equal to L n for obligor n, the portfolio loss distribution can then be derived as, ( N ) P(L l) = s n L n d n P(D 1 = d 1,..., D N = d N ). (d 1,...,d N ) {0,1} N P N n=1 sn Ln dn l n=1 Impractical from a computational point of view for realistic portfolios (for instance N = 1000). Remark 1.1 We present an analytical approximation for the α th percentile of the loss distribution in the one-factor framework, under the assumption that portfolios are infinitely fine-grained such that the idiosyncratic risk is completely diversified. Luis Ortiz-Gracia (CRM and CWI) CRM-Empresa 2013 Concentration Risk Measurement with Haar Wavelets 11 / 38

12 The ASRF Model Introduction General Model Settings The Merton Model The ASRF Model and Concentration Risk The Asymptotic Single Risk Factor Model (ASRF) is the model chosen in Basel II to calculate regulatory capital. It is based on the one-factor Merton model and it mainly relies in the following assumptions, Assumption Portfolios are infinitely fine-grained, i.e. no exposure accounts for more than an arbitrarily small share of total portfolio exposure. 2 Dependence across exposures is driven by a single systematic risk factor Y. Default indicators are mutually independent conditional on Y. Assumption N n=1 E n. 2 There exist a positive ζ such that the largest exposure share is of order O(N ( 1 2 +ζ) ). Luis Ortiz-Gracia (CRM and CWI) CRM-Empresa 2013 Concentration Risk Measurement with Haar Wavelets 12 / 38

13 Introduction General Model Settings The Merton Model The ASRF Model and Concentration Risk Theorem 1.1 Let us denote the exposure share of obligor n by s n = P En N. Then, n=1 En under assumptions 1.2 and 1.3 the portfolio loss ratio L = N n=1 s n L n D n conditional on any realization y of the systematic risk factor Y satisfies, L E(L Y = y) 0 almost surely as N. Under one-factor Merton model and assuming L n to be deterministic, N ( tn ) ρ n y E (L Y = y) = s n L n Φ. (1) 1 ρn n=1 By Theorem 1.1: VaR α (L) E(L Y = l 1 α (Y )) 0 a.s. as N. Finally, VaR A α = ( N n=1 s tn+ n L n Φ ) ρ nφ 1 (α) 1 ρn and, VaRC A α,n = s n VaRA α s n = s n L n Φ ( ) tn+ ρ nφ 1 (α) 1 ρn. Luis Ortiz-Gracia (CRM and CWI) CRM-Empresa 2013 Concentration Risk Measurement with Haar Wavelets 13 / 38

14 Concentration Risk Introduction General Model Settings The Merton Model The ASRF Model and Concentration Risk However: Real world portfolios are not perfectly fine-grained. The ASRF model might be approximately valid for huge portfolios but less satisfactory for portfolios of smaller institutions (or more specialized). The formula can underestimate the required economic capital. Does not allow the measurement of sector concentration risk. In practice: Monte Carlo simulations (robust but computationally intensive). Proposal: A new method based on wavelets to overcome the computational complexity. References: E. Lütkebohmert (2009). Concentration Risk in Credit Portfolios. Springer. Luis Ortiz-Gracia (CRM and CWI) CRM-Empresa 2013 Concentration Risk Measurement with Haar Wavelets 14 / 38

15 Outline Numerical Analysis of Wavelet Methods Laplace Transform Inversion: the WA Method 1 Portfolio Credit Risk Modeling Luis Ortiz-Gracia (CRM and CWI) CRM-Empresa 2013 Concentration Risk Measurement with Haar Wavelets 15 / 38

16 Introduction Numerical Analysis of Wavelet Methods Laplace Transform Inversion: the WA Method Multi-scale methods: signal analysis, statistics, image processing and numerical analysis. Approximations (f j ) j 0 to the unknown function f at various resolution levels indexed by j. Wavelet: a little wave with remarkable approximation properties. Fourier basis: composed of waves (approximation in frequency domain). Wavelet basis: composed of wavelets (approximation in frequency and time domain) x Luis Ortiz-Gracia (CRM and CWI) CRM-Empresa 2013 Concentration Risk Measurement with Haar Wavelets 16 / 38

17 The Haar System Numerical Analysis of Wavelet Methods Laplace Transform Inversion: the WA Method Example: the step function 1, x [ 1 2, 0], f (x) = 1, x (0, 1 2 ], 0, otherwise. This function is poorly approximated by its Fourier series: 1.5 Step function Fourier (5 coefficients) 1.5 Step function Fourier (50 coefficients) x x Wavelets are more flexible: f (x) = 2 2 φ 1,0(x) 2 2 φ 1, 1(x). References: I. Daubechies (1992). Ten Lectures on Wavelets. SIAM. Luis Ortiz-Gracia (CRM and CWI) CRM-Empresa 2013 Concentration Risk Measurement with Haar Wavelets 17 / 38

18 The WA Method Numerical Analysis of Wavelet Methods Laplace Transform Inversion: the WA Method Let f be a function in L 2 ([0, 1]). f (x) = 2 m 1 k=0 or alternatively, f (x) = lim m + f m (x), c m,k = k+1 2 m c m,k φ m,k (x) j 1 d j,k ψ j,k (x), j=m k=0 f m (x) = 2 m 1 k=0 c m,kφ m,k (x), where, f (x)φ m,k (x)dx, d j,k = k k+1 2 m f (x)ψ j,k (x)dx, k 2 m k = 0,..., 2 m 1, j m, k = 0,, 2 j 1 and {φ m,k } k=0,...,2m 1 {ψ j,k } j m,k=0,,2j 1 is the Haar basis system in L 2 ([0, 1]). Luis Ortiz-Gracia (CRM and CWI) CRM-Empresa 2013 Concentration Risk Measurement with Haar Wavelets 18 / 38

19 The WA Method Numerical Analysis of Wavelet Methods Laplace Transform Inversion: the WA Method Consider the Laplace Transform of f : f (s) = + 0 e sx f (x)dx, (assume f (x) = 0, x / [0, 1]). Wavelet Approximation (WA) method: approximate f by f m and compute the coefficients c m,k. + f (s) = e sx f (x)dx 0 = 2m/2 s ( 1 e s m ) 2 m 1 k=0 e sx f m (x)dx = c m,k e s k 2 m. Change of variable z = e s 1 2 m : 2 m 1 k=0 c m,kz k Q m (z). Luis Ortiz-Gracia (CRM and CWI) CRM-Empresa 2013 Concentration Risk Measurement with Haar Wavelets 19 / 38

20 The WA Method Numerical Analysis of Wavelet Methods Laplace Transform Inversion: the WA Method We obtain the coefficients c m,k by means of the Cauchy s integral formula, c m,k 1 Q m (z) 2πi γ z k+1 dz, k = 0,..., 2m 1, where γ denotes a circle of radius r, 0 < r < 1, about the origin. Considering now the change of variable z = re iu, 0 < r < 1 we have, c m,k 2 πr k π 0 R(Q m (re iu )) cos(ku)du, k = 0,..., 2 m 1. Finally, the integral can be evaluated by means of the trapezoidal rule. Luis Ortiz-Gracia (CRM and CWI) CRM-Empresa 2013 Concentration Risk Measurement with Haar Wavelets 20 / 38

21 Outline VaR Computation with the WA Method Numerical Examples 1 Portfolio Credit Risk Modeling Luis Ortiz-Gracia (CRM and CWI) CRM-Empresa 2013 Concentration Risk Measurement with Haar Wavelets 21 / 38

22 The Model VaR Computation with the WA Method Numerical Examples Focus on the one-factor Merton model. Assume: L n = 100% and N n=1 E n = 1. Let F be the CDF of L and f L its PDF. r n = ρy + 1 ρɛ n (Y, ɛ n i.i.d. N(0, 1)). ( tn Conditional default probabilities, P n (y) Φ ρy 1 ρ ), t n = Φ 1 (P n ). References: Granularity Adjustment: Gordy and Lütkebohmert (2007). Recursive Approximation: Andersen et al. (2003). Normal Approximation: Martin (2004). Saddle Point: Martin et al. (2001), Huang and Oosterlee (2007) (cond. framework). Poisson Method: Glasserman (2007). Luis Ortiz-Gracia (CRM and CWI) CRM-Empresa 2013 Concentration Risk Measurement with Haar Wavelets 22 / 38

23 The Approximation VaR Computation with the WA Method Numerical Examples Consider, F (x) = { F (x), if 0 x 1, 1, if x > 1, Define unconditional MGF: ML (s) E(e sl ). Assumption 3.1 Conditional Independence Framework. If the systematic factor Y is fixed, defaults occur independently because the only remaining uncertainty is the idiosyncratic risk. Define conditional MGF: M L (s; y) E(e sl Y = y) = N [ ] n=1 1 Pn (y) + P n (y)e sen. Then: M L (s) = E( ML (s; y)) = N [ ] R n=1 1 Pn (y) + P n (y)e sen 1 2π e y2 2 dy. Luis Ortiz-Gracia (CRM and CWI) CRM-Empresa 2013 Concentration Risk Measurement with Haar Wavelets 23 / 38

24 The Approximation VaR Computation with the WA Method Numerical Examples Since: F L 2 ([0, 1]) then: F (x) F m (x), F m (x) = 2 m 1 k=0 c m,kφ m,k (x), F (x) = lim m + F m (x). Observe: ML (s) = + e sx F (x)dx = e s + s e sx F (x)dx. ( ) Then: M L (s) e s /s is the Laplace transform of F. Apply the WA method, Compute: c m,k 2 π πr k 0 R(Q m(re iu )) cos(ku)du, k = 0,..., 2 m 1, by means of the trapezoidal rule. Luis Ortiz-Gracia (CRM and CWI) CRM-Empresa 2013 Concentration Risk Measurement with Haar Wavelets 24 / 38

25 VaR Computation with the WA Method Numerical Examples Parameters: m = 10, m T = 2 m, l = 20. MC with scenarios. Portfolio 3.1 We consider N = 102 obligors, with P n = 0.1%, E n = 1, n = 1,..., 100, E 101 = E 102 = 20, ρ = 0.3 and L n = 1. Method VaR Relative Error Monte Carlo ASRF % Saddle Point % Wavelet Approximation % Luis Ortiz-Gracia (CRM and CWI) CRM-Empresa 2013 Concentration Risk Measurement with Haar Wavelets 25 / 38

26 VaR Computation with the WA Method Numerical Examples 1e-02 Saddle Point Monte Carlo ASRF Tail probability 1e-03 1e Loss level Figure: Tail probability approximation of a heterogeneous portfolio with severe name concentration. Luis Ortiz-Gracia (CRM and CWI) CRM-Empresa 2013 Concentration Risk Measurement with Haar Wavelets 26 / 38

27 VaR Computation with the WA Method Numerical Examples 1 Portfolio N P n E n ρ HHI N P % C n P % C n P % C n P % C n P % N P % C n Portfolio VaR W (8) RE(0.999, 8) VaR W (9) RE(0.999, 9) VaR W (10) RE(0.999, 10) VaR M P % % % P % % % P % % % P % % % Luis Ortiz-Gracia (CRM and CWI) CRM-Empresa 2013 Concentration Risk Measurement with Haar Wavelets 27 / 38

28 VaR Computation with the WA Method Numerical Examples Portfolio VaR W (8) VaR W (9) VaR W (10) VaR M P P P P Table: CPU time (in seconds). Portfolio VaR W (10) RE(0.9999, 10) VaR W (10) RE( , 10) P % % P % % P % % P % % 2 11 Portfolio VaR W (10) RE(0.9999, 10) VaR W (10) RE( , 10) P % % P % % P % % P % % MC 2 10 Portfolio VaR M VaR M P P P P Luis Ortiz-Gracia (CRM and CWI) CRM-Empresa 2013 Concentration Risk Measurement with Haar Wavelets 28 / 38

29 VaR Computation with the WA Method Numerical Examples 1e+00 Wavelet Approximation (scale 10) Monte Carlo ASRF 1e-01 Tail probability 1e-02 1e-03 1e Loss level Figure: Tail probability approximation of Portfolios P1 at scale m = 10. Luis Ortiz-Gracia (CRM and CWI) CRM-Empresa 2013 Concentration Risk Measurement with Haar Wavelets 29 / 38

30 VaR Computation with the WA Method Numerical Examples 1e+00 Wavelet Approximation (scale 10) Monte Carlo ASRF 1e-01 Tail probability 1e-02 1e-03 1e Loss level Figure: Tail probability approximation of Portfolios P2 at scale m = 10. Luis Ortiz-Gracia (CRM and CWI) CRM-Empresa 2013 Concentration Risk Measurement with Haar Wavelets 30 / 38

31 VaR Computation with the WA Method Numerical Examples 1e+00 Wavelet Approximation (scale 10) Monte Carlo ASRF 1e-01 Tail probability 1e-02 1e-03 1e Loss level Figure: Tail probability approximation of Portfolios P3 at scale m = 10. Luis Ortiz-Gracia (CRM and CWI) CRM-Empresa 2013 Concentration Risk Measurement with Haar Wavelets 31 / 38

32 VaR Computation with the WA Method Numerical Examples 1e+00 Wavelet Approximation (scale 10) Monte Carlo ASRF 1e-01 Tail probability 1e-02 1e-03 1e Loss level Figure: Tail probability approximation of Portfolios P4 at scale m = 10. Luis Ortiz-Gracia (CRM and CWI) CRM-Empresa 2013 Concentration Risk Measurement with Haar Wavelets 32 / 38

33 Outline 1 Portfolio Credit Risk Modeling Luis Ortiz-Gracia (CRM and CWI) CRM-Empresa 2013 Concentration Risk Measurement with Haar Wavelets 33 / 38

34 European Options We consider the risk-neutral valuation formula, v(x, t) = e r(t t) E Q (v(y, T ) x) = e r(t t) R v(y, T )f (y x)dy, (2) Whereas f is typically not known, the characteristic function of the log-asset price is often known. We represent the payoff as a function of the log-asset price, and denote the log-asset prices by, x = log(s 0 /K) and y = log(s T /K), with S t the underlying price at time t and K the strike price. The payoff v(y, T ) for European options in log-asset price then reads, { v(y, T ) = [α K (e y 1)] + 1, for a call,, with, α = 1, for a put. Luis Ortiz-Gracia (CRM and CWI) CRM-Empresa 2013 Concentration Risk Measurement with Haar Wavelets 34 / 38

35 Outline 1 Portfolio Credit Risk Modeling Luis Ortiz-Gracia (CRM and CWI) CRM-Empresa 2013 Concentration Risk Measurement with Haar Wavelets 35 / 38

36 New method for Laplace Transform inversion based on Haar wavelets. Particularly well suited for stepped-shape functions, often arising in discrete probability models. Computation of the VaR risk measure under the one-factor Merton model. Very accurate and fast: MC/WA 300, even in the presence of severe name concentration. Rel. err. < 1%. The WA method computes the entire distribution of losses without extra computational time (for instance CDO pricing). The WA method can be extended to compute the Expected Shortfall and the Risk Contributions to VaR and ES. Luis Ortiz-Gracia (CRM and CWI) CRM-Empresa 2013 Concentration Risk Measurement with Haar Wavelets 36 / 38

37 References J. J. Masdemont and L. Ortiz-Gracia (2011). Haar wavelets-based approach for quantifying credit portfolio losses. Quantitative Finance, DOI: / L. Ortiz-Gracia and J. J. Masdemont (2012). Credit risk contributions under the Vasicek one-factor model: a fast wavelet expansion approximation. To appear in Journal of Computational Finance. L. Ortiz-Gracia and C. W. Oosterlee (2013). Robust pricing of European options with wavelets and the characteristic function. Submitted. Luis Ortiz-Gracia (CRM and CWI) CRM-Empresa 2013 Concentration Risk Measurement with Haar Wavelets 37 / 38

38 Thank you for your attention Luis Ortiz-Gracia (CRM and CWI) CRM-Empresa 2013 Concentration Risk Measurement with Haar Wavelets 38 / 38