# HIERARCHICAL MODELS IN EXTREME VALUE THEORY

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1 HIERARCHICAL MODELS IN EXTREME VALUE THEORY Richard L. Smith Department of Statistics and Operations Research, University of North Carolina, Chapel Hill and Statistical and Applied Mathematical Sciences Institute Department of Statistics North Carolina State University February 19, 2015

2 OUTLINE OF TALKs I. An Example of Hierarchical Models Applied to Insurance Extremes II. Attribution of Climate Extremes III. The Attribution Problem for Joint Distributions of Climate Extremes (introduction)

3 I. AN EXAMPLE OF HIERARCHICAL MODELS APPLIED TO INSURANCE EXTREMES From the book chapter Bayesian Risk Analysis by R.L. Smith and D.J. Goodman (2000) See also: R.L. Smith (2003), Statistics of Extremes, With Applications in Environment, Insurance and Finance. In Extreme Values in Finance, Telecommunications and the Environment, edited by B. Finkenstadt and H. Rootzen, Chapman and Hall/CRC Press, London, pp

4 The data consist of all insurance claims experienced by a large international oil company over a threshold 0.5 during a 15-year period a total of 393 claims. Seven types: Type Description Number Mean 1 Fire Liability Offshore Cargo Hull Onshore Aviation Total of all 393 claims: largest claims: 776.2, 268.0, 142.0, 131.0, 95.8, 56.8, 46.2, 45.2, 40.4, 30.7.

5 (a) (b) Claim Size Total number of claims Years From Start Years From Start (c) (d) Total size of claims Mean excess over threshold Years From Start Threshold Some plots of the insurance data.

6 Some problems: 1. What is the distribution of very large claims? 2. Is there any evidence of a change of the distribution over time? 3. What is the influence of the different types of claim? 4. How should one characterize the risk to the company? More precisely, what probability distribution can one put on the amount of money that the company will have to pay out in settlement of large insurance claims over a future time period of, say, three years?

7 Introduction to Univariate Extreme Value Theory

8 EXTREME VALUE DISTRIBUTIONS X 1, X 2,..., i.i.d., F (x) = Pr{X i x}, M n = max(x 1,..., X n ), Pr{M n x} = F (x) n. For non-trivial results must renormalize: find a n > 0, b n such that { } Mn b n Pr x = F (a n x + b n ) n H(x). a n The Three Types Theorem (Fisher-Tippett, Gnedenko) asserts that if nondegenerate H exists, it must be one of three types: H(x) = exp( e x ), all x (Gumbel) { 0 x < 0 H(x) = exp( x α ) x > 0 (Fréchet) { exp( x H(x) = α ) x < 0 1 x > 0 (Weibull) In Fréchet and Weibull, α > 0.

9 The three types may be combined into a single generalized extreme value (GEV) distribution: ( H(x) = exp 1 + ξ x µ ) 1/ξ ψ, (y + = max(y, 0)) where µ is a location parameter, ψ > 0 is a scale parameter and ξ is a shape parameter. ξ 0 corresponds to the Gumbel distribution, ξ > 0 to the Fréchet distribution with α = 1/ξ, ξ < 0 to the Weibull distribution with α = 1/ξ. ξ > 0: long-tailed case, 1 F (x) x 1/ξ, ξ = 0: exponential tail ξ < 0: short-tailed case, finite endpoint at µ ξ/ψ +

10 EXCEEDANCES OVER THRESHOLDS Consider the distribution of X conditionally on exceeding some high threshold u: F u (y) = F (u + y) F (u). 1 F (u) As u ω F = sup{x : F (x) < 1}, often find a limit F u (y) G(y; σ u, ξ) where G is generalized Pareto distribution (GPD) G(y; σ, ξ) = 1 ( 1 + ξ y ) 1/ξ σ + Equivalence to three types theorem established by Pickands (1975)..

11 The Generalized Pareto Distribution G(y; σ, ξ) = 1 ( 1 + ξ y ) 1/ξ σ +. ξ > 0: long-tailed (equivalent to usual Pareto distribution), tail like x 1/ξ, ξ = 0: take limit as ξ 0 to get G(y; σ, 0) = 1 exp i.e. exponential distribution with mean σ, ( y ), σ ξ < 0: finite upper endpoint at σ/ξ.

12 POISSON-GPD MODEL FOR EXCEEDANCES 1. The number, N, of exceedances of the level u in any one year has a Poisson distribution with mean λ, 2. Conditionally on N 1, the excess values Y 1,..., Y N are IID from the GPD.

13 Relation to GEV for annual maxima: Suppose x > u. The probability that the annual maximum of the Poisson-GPD process is less than x is Pr{ max Y i x} = Pr{N = 0} + 1 i N = e λ + = exp { n=1 ( λ λ n e λ n! { ξ x u σ ( 1 + ξ x u σ ) 1/ξ }. n=1 ) } 1/ξ n Pr{N = n, Y 1 x,... Y n x} This is GEV with σ = ψ + ξ(u µ), λ = ( 1 + ξ u µ ) 1/ξ ψ. Thus the GEV and GPD models are entirely consistent with one another above the GPD threshold, and moreover, shows exactly how the Poisson GPD parameters σ and λ vary with u.

14 ALTERNATIVE PROBABILITY MODELS 1. The r largest order statistics model If Y n,1 Y n,2... Y n,r are r largest order statistics of IID sample of size n, and a n and b n are EVT normalizing constants, then ( Yn,1 b n,..., Y ) n,r b n a n a n converges in distribution to a limiting random vector (X 1,..., X r ), whose density is ( h(x 1,..., x r ) = ψ r exp 1 + ξ x ) 1/ξ r µ ψ ( ξ ) r j=1 log ( 1 + ξ x j µ ψ ).

15 2. Point process approach (Smith 1989) Two-dimensional plot of exceedance times and exceedance levels forms a nonhomogeneous Poisson process with Λ(A) = (t 2 t 1 )Ψ(y; µ, ψ, ξ) Ψ(y; µ, ψ, ξ) = ( 1 + ξ y µ ψ ) 1/ξ (1 + ξ(y µ)/ψ > 0).

16 Illustration of point process model.

17 An extension of this approach allows for nonstationary processes in which the parameters µ, ψ and ξ are all allowed to be timedependent, denoted µ t, ψ t and ξ t. This is the basis of the extreme value regression approaches introduced later Comment. The point process approach is almost equivalent to the following: assume the GEV (not GPD) distribution is valid for exceedances over the threshold, and that all observations under the threshold are censored. Compared with the GPD approach, the parameterization directly in terms of µ, ψ, ξ is often easier to interpret, especially when trends are involved.

18 GEV log likelihood: l Y (µ, ψ, ξ) = N log ψ ESTIMATION i ( ( ) 1 ξ ξ Y i µ ψ i ) 1/ξ provided 1 + ξ(y i µ)/ψ > 0 for each i. log ( 1 + ξ Y i µ ψ ) Poisson-GPD model: l N,Y (λ, σ, ξ) = N log λ λt N log σ provided 1 + ξy i /σ > 0 for all i. ( ξ ) N i=1 log ( 1 + ξ Y i σ ) Usual asymptotics valid if ξ > 1 2 (Smith 1985)

19 Bayesian approaches An alternative approach to extreme value inference is Bayesian, using vague priors for the GEV parameters and MCMC samples for the computations. Bayesian methods are particularly useful for predictive inference, e.g. if Z is some as yet unobserved random variable whose distribution depends on µ, ψ and ξ, estimate Pr{Z > z} by Pr{Z > z; µ, ψ, ξ}π(µ, ψ, ξ Y )dµdψdξ where π(... Y ) denotes the posterior density given past data Y

20 Plots of women s 3000 meter records, and profile log-likelihood for ultimate best value based on pre-1993 data.

21 Example. The left figure shows the five best running times by different athletes in the women s 3000 metre track event for each year from 1972 to Also shown on the plot is Wang Junxia s world record from Many questions were raised about possible illegal drug use. We approach this by asking how implausible Wang s performance was, given all data up to Robinson and Tawn (1995) used the r largest order statistics method (with r = 5, translated to smallest order statistics) to estimate an extreme value distribution, and hence computed a profile likelihood for x ult, the lower endpoint of the distribution, based on data up to 1992 (right plot of previous figure)

22 Alternative Bayesian calculation: (Smith 1997) Compute the (Bayesian) predictive probability that the 1993 performance is equal or better to Wang s, given the data up to 1992, and conditional on the event that there is a new world record. The answer is approximately

23 Insurance Extremes Dataset We return to the oil company data set discussed earlier. Prior to any of the analysis, some examination was made of clustering phenomena, but this only reduced the original 425 claims to 393 independent claims (Smith & Goodman 2000) GPD fits to various thresholds: u N u Mean σ ξ Excess

24 Point process approach: u N u µ log ψ ξ (4.4) (0.24) (0.09) (5.2) (0.31) (0.16) (5.5) (0.31) (0.21) (5.7) (0.32) (0.25) (3.9) (0.46) (0.45) (5.7) (0.56) (0.53) (8.6) (0.66) (0.56) Standard errors are in parentheses

25 Predictive Distributions of Future Losses What is the probability distribution of future losses over a specific time period, say 1 year? Let Y be future total loss. Distribution function G(y; µ, ψ, ξ) in practice this must itself be simulated.

26 Traditional frequentist approach: where ˆµ, ˆψ, ˆξ are MLEs. Ĝ(y) = G(y; ˆµ, ˆψ, ˆξ) Bayesian: G(y) = G(y; µ, ψ, ξ)dπ(µ, ψ, ξ X) where π( X) denotes posterior density given data X.

27 (a) (b) post.density post.density mu Log psi (c) (d) post.density Amount of Loss xi N=1/(probability of loss) Estimated posterior densities for the three parameters, and for the predictive distribution function. Four independent Monte Carlo runs are shown for each plot.

28 Hierarchical models for claim type and year effects Further features of the data: 1. When separate GPDs are fitted to each of the 6 main types, there are clear differences among the parameters. 2. The rate of high-threshold crossings does not appear uniform, but peaks around years

29 A Hierarchical Model: Level I. Parameters m µ, m ψ, m ξ, s 2 µ, s 2 ψ, s2 ξ a prior distribution. are generated from Level II. Conditional on the parameters in Level I, parameters µ 1,..., µ J (where J is the number of types) are independently drawn from N(m µ, s 2 µ), the normal distribution with mean m µ, variance s 2 µ. Similarly, log ψ 1,..., log ψ J are drawn independently from N(m ψ, s 2 ψ ), ξ 1,..., ξ J are drawn independently from N(m ξ, s 2 ξ ). Level III. Conditional on Level II, for each j {1,..., J}, the point process of exceedances of type j is generated from the Poisson process with parameters µ j, ψ j, ξ j.

30 This model may be further extended to include a year effect, as follows. Suppose the extreme value parameters for type j in year k are not µ j, ψ j, ξ j but µ j + δ k, ψ j, ξ j. We fix δ 1 = 0 to ensure identifiability, and let {δ k, k > 1} follow an AR(1) process: with a vague prior on (ρ, s 2 η). δ k = ρδ k 1 + η k, η k N(0, s 2 η) We show boxplots for each of µ j, log ψ j, ξ j, j = 1,..., 6 and for δ k, k = 2, 15.

31 (a) (b) mu log psi type type (c) (d) xi delta type Posterior means, quartiles for µ j, log ψ j, ξ j (j = 1,..., 6) and δ k (k = 2,..., 15). year

32 50000 A: All data combined B: Separate types A C: Separate types and years D: As C, outliers omitted B C Amount of loss 5000 D N=1/(probability of loss) Posterior predictive distribution functions (log-log scale) for homogenous model (curve A) and three versions of hierarchical model

33 II. ATTRIBUTION OF CLIMATE EXTREMES (Joint work with Michael Wehner, Lawrence Berkeley National Laboratory)

34 Superstorm Sandy on October (Scott Sistek)

35 Superstorm Sandy ( October 30, 2012)

36 Superstorm Sandy ( October 31, 2012)

37

38 Motivating Question: Concern over increasing frequency of extreme meteorological events Is the increasing frequency a result of anthropogenic influence? How much more rapidly with they increase in the future? Focus on three specific events: heatwaves in Europe 2003, Russia 2010 and Central USA 2011 Identify meteorological variables of interest JJA temperature averages over a region Europe 10 o W to 40 o E, 30 o to 50 o N Russia 30 o to 60 o E, 45 o to 65 o N Central USA 90 o to 105 o W, 25 o to 45 o N Probabilities of crossing thresholds respectively 1.92K, 3.65K, 2.01K in any year from 1990 to 2040.

39 Background Stott, Stone and Allen (2004) defined fraction of attributable risk (FAR) Observe some extreme event Let P 1 be probability of this event estimated from models including anthropogenic forcings, P 0 corresponding probability under natural forcings F AR = 1 P 0 P (also consider RR = P 1 1 P ) 0 For the Europe 2003 event they claimed P , P 0 1 so RR = 4 and F AR = = Very likely (confidence level at least 90%) that the F AR was at least 0.5 (RR 2), Method used a combination of extreme value theory, and detection and attribution methodology from the climate literature

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