Generalized Pareto distribution
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| Generalized Pareto distribution | |||
|---|---|---|---|
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Probability density function Gpdpdf GPD distribution functions for {\displaystyle \mu =0} and different values of {\displaystyle \sigma } and {\displaystyle \xi } | |||
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Cumulative distribution function Gpdcdf | |||
| Parameters |
{\displaystyle \mu \in (-\infty ,\infty ),円} location (real) | ||
| Support |
{\displaystyle x\geq \mu ,円\;(\xi \geq 0)} | ||
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{\displaystyle {\frac {1}{\sigma }}(1+\xi z)^{-(1/\xi +1)}} | |||
| CDF | {\displaystyle 1-(1+\xi z)^{-1/\xi },円} | ||
| Mean | {\displaystyle \mu +{\frac {\sigma }{1-\xi }},円\;(\xi <1)} | ||
| Median | {\displaystyle \mu +{\frac {\sigma (2^{\xi }-1)}{\xi }}} | ||
| Mode | {\displaystyle \mu } | ||
| Variance | {\displaystyle {\frac {\sigma ^{2}}{\left(1-\xi \right)^{2}(1-2\xi )}},円\;(\xi <1/2)} | ||
| Skewness | {\displaystyle {\frac {2(1+\xi ){\sqrt {1-2\xi }}}{(1-3\xi )}},円\;(\xi <1/3)} | ||
| Excess kurtosis | {\displaystyle {\frac {3(1-2\xi )(2\xi ^{2}+\xi +3)}{(1-3\xi )(1-4\xi )}}-3,円\;(\xi <1/4)} | ||
| Entropy | {\displaystyle \log(\sigma )+\xi +1} | ||
| MGF | {\displaystyle e^{\theta \mu },円\sum _{j=0}^{\infty }\left[{\frac {(\theta \sigma )^{j}}{\prod _{k=0}^{j}(1-k\xi )}}\right],\;(k\xi <1)} | ||
| CF | {\displaystyle e^{it\mu },円\sum _{j=0}^{\infty }\left[{\frac {(it\sigma )^{j}}{\prod _{k=0}^{j}(1-k\xi )}}\right],\;(k\xi <1)} | ||
| Method of moments |
{\displaystyle \xi ={\frac {1}{2}}\left(1-{\frac {\left(\operatorname {E} [X]-\mu \right)^{2}}{\operatorname {Var} [X]}}\right)} {\displaystyle \sigma =(\operatorname {E} [X]-\mu )(1-\xi )} | ||
| Expected shortfall | {\displaystyle {\begin{cases}\mu +\sigma \left[{\frac {(1-p)^{-\xi }}{1-\xi }}+{\frac {(1-p)^{-\xi }-1}{\xi }}\right],&\xi \neq 0\\\mu +\sigma [1-\ln(1-p)],&\xi =0\end{cases}}}[1] | ||
In statistics, the generalized Pareto distribution (GPD) is a family of continuous probability distributions. It is often used to model the tails of another distribution. It is specified by three parameters: location {\displaystyle \mu }, scale {\displaystyle \sigma }, and shape {\displaystyle \xi }.[2] [3] Sometimes it is specified by only scale and shape[4] and sometimes only by its shape parameter. Some references give the shape parameter as {\displaystyle \kappa =-\xi ,円}.[5]
With shape {\displaystyle \xi >0} and location {\displaystyle \mu =\sigma /\xi }, the GPD is equivalent to the Pareto distribution with scale {\displaystyle x_{m}=\sigma /\xi } and shape {\displaystyle \alpha =1/\xi }.
Definition
[edit ]The cumulative distribution function of {\displaystyle X\sim {\text{GPD}}(\mu ,\sigma ,\xi )} ({\displaystyle \mu \in \mathbb {R} }, {\displaystyle \sigma >0}, and {\displaystyle \xi \in \mathbb {R} }) is
{\displaystyle F_{(\mu ,\sigma ,\xi )}(x)={\begin{cases}1-\left(1+{\frac {\xi (x-\mu )}{\sigma }}\right)^{-1/\xi }&{\text{for }}\xi \neq 0,\1円-\exp \left(-{\frac {x-\mu }{\sigma }}\right)&{\text{for }}\xi =0,\end{cases}}} where the support of {\displaystyle X} is {\displaystyle x\geq \mu } when {\displaystyle \xi \geq 0,円}, and {\displaystyle \mu \leq x\leq \mu -\sigma /\xi } when {\displaystyle \xi <0}.
The probability density function (pdf) of {\displaystyle X\sim {\text{GPD}}(\mu ,\sigma ,\xi )} is
{\displaystyle f_{(\mu ,\sigma ,\xi )}(x)={\frac {1}{\sigma }}\left(1+{\frac {\xi (x-\mu )}{\sigma }}\right)^{-\left(1+1/\xi \right)},}
again, for {\displaystyle x\geq \mu } when {\displaystyle \xi \geq 0}, and {\displaystyle \mu \leq x\leq \mu -\sigma /\xi } when {\displaystyle \xi <0}.
The pdf is a solution of the following differential equation: [citation needed ]
{\displaystyle {\begin{cases}f'(x)\left(-\mu \xi +\sigma +\xi x\right)+(\xi +1)f(x)=0,\\[1ex]f(0)={\frac {1}{\sigma }}\left(1-{\frac {\mu \xi }{\sigma }}\right)^{-{\frac {1}{\xi }}-1}\end{cases}}}
The standard cumulative distribution function (cdf) of the GPD is defined using {\displaystyle z={\frac {x-\mu }{\sigma }}.}[6]
{\displaystyle F_{\xi }(z)={\begin{cases}1-\left(1+\xi z\right)^{-1/\xi }&{\text{for }}\xi \neq 0,\1円-e^{-z}&{\text{for }}\xi =0.\end{cases}}}
where the support is {\displaystyle z\geq 0} for {\displaystyle \xi \geq 0} and {\displaystyle 0\leq z\leq -1/\xi } for {\displaystyle \xi <0}. The corresponding probability density function (pdf) is
{\displaystyle f_{\xi }(z)={\begin{cases}\left(1+\xi z\right)^{-(1+1/\xi )}&{\text{for }}\xi \neq 0,\\e^{-z}&{\text{for }}\xi =0.\end{cases}}}
Special cases
[edit ]- If {\displaystyle \xi =0}, the GPD is the exponential distribution.
- If {\displaystyle \xi >0}, the GPD is the Pareto distribution with shape {\displaystyle \alpha =1/\xi }.
- If {\displaystyle \xi <0}, the GPD is the power function distribution with shape {\displaystyle \alpha =-1/\xi }.
- If {\displaystyle \xi =-1}, the GPD is the continuous uniform distribution {\displaystyle U(0,\sigma )}.[7]
- If {\displaystyle X\sim \mathrm {GPD} (\mu =0,\sigma ,\xi )}, then {\displaystyle Y=\log(X)\sim \mathrm {exGPD} (\sigma ,\xi )} [1]. (exGPD stands for the exponentiated generalized Pareto distribution.)
- GPD is similar to the Burr distribution.
Prediction
[edit ]- It is often of interest to predict probabilities of out-of-sample data under the assumption that both the training data and the out-of-sample data follow a GPD.
- Predictions of probabilities generated by substituting maximum likelihood estimates of the GPD parameters into the cumulative distribution function ignore parameter uncertainty. As a result, the probabilities are not well calibrated, do not reflect the frequencies of out-of-sample events, and, in particular, underestimate the probabilities of out-of-sample tail events.[8]
- Predictions generated using the objective Bayesian approach of calibrating prior prediction have been shown to greatly reduce this underestimation, although not completely eliminate it.[8] Calibrating prior prediction is implemented in the R software package fitdistcp.[2]
Generating generalized Pareto random variables
[edit ]Generating GPD random variables
[edit ]If U is uniformly distributed on (0, 1], then
{\displaystyle X=\mu +{\frac {\sigma (U^{-\xi }-1)}{\xi }}\sim \mathrm {GPD} (\mu ,\sigma ,\xi \neq 0)} and {\displaystyle X=\mu -\sigma \ln(U)\sim \mathrm {GPD} (\mu ,\sigma ,\xi =0).}
Both formulas are obtained by inversion of the cdf.
The Pareto package in R and the gprnd command in the Matlab Statistics Toolbox can be used to generate generalized Pareto random numbers.
GPD as an Exponential-Gamma Mixture
[edit ]A GPD random variable can also be expressed as an exponential random variable, with a Gamma distributed rate parameter.
{\displaystyle X\mid \Lambda \sim \mathrm {Exp} (\Lambda )} and {\displaystyle \Lambda \sim \mathrm {Gamma} (\alpha ,,円\beta )} then {\displaystyle X\sim \mathrm {GPD} (\xi =1/\alpha ,\ \sigma =\beta /\alpha )}
Notice however, that since the parameters for the Gamma distribution must be greater than zero, we obtain the additional restrictions that {\displaystyle \xi } must be positive.
In addition to this mixture (or compound) expression, the generalized Pareto distribution can also be expressed as a simple ratio. Concretely, for {\displaystyle Y\sim \mathrm {Exp} (1)} and {\displaystyle Z\sim \mathrm {Gamma} (1/\xi ,,1円),,円} we have {\displaystyle \mu +{\frac {\sigma Y}{\xi Z}}\sim \mathrm {GPD} (\mu ,\sigma ,\xi ),円.} This is a consequence of the mixture after setting {\displaystyle \beta =\alpha } and taking into account that the rate parameters of the exponential and gamma distribution are simply inverse multiplicative constants.
Exponentiated generalized Pareto distribution
[edit ]The exponentiated generalized Pareto distribution (exGPD)
[edit ]If {\displaystyle X\sim \mathrm {GPD} (\mu =0,\sigma ,\xi )}, then {\displaystyle Y=\log(X)} is distributed according to the exponentiated generalized Pareto distribution, denoted by {\displaystyle Y\sim \mathrm {exGPD} (\sigma ,\xi )}.
The probability density function(pdf) of {\displaystyle Y\sim \mathrm {exGPD} (\sigma ,\xi ),円,円(\sigma >0)} is
{\displaystyle g_{(\sigma ,\xi )}(y)={\begin{cases}{\frac {e^{y}}{\sigma }}{\bigg (}1+{\frac {\xi e^{y}}{\sigma }}{\bigg )}^{-1/\xi -1},円,円,円,円{\text{for }}\xi \neq 0,\\{\frac {1}{\sigma }}e^{y-e^{y}/\sigma },円,円,円,円,円,円,円,円,円,円,円,円,円,円,円,円,円,円,円,円,円,円,円,円,円,円,円,円,円,円,円,円{\text{for }}\xi =0,\end{cases}}} where the support is {\displaystyle -\infty <y<\infty } for {\displaystyle \xi \geq 0}, and {\displaystyle -\infty <y\leq \log(-\sigma /\xi )} for {\displaystyle \xi <0}.
For all {\displaystyle \xi }, the {\displaystyle \log \sigma } becomes the location parameter. See the right panel for the pdf when the shape {\displaystyle \xi } is positive.
The exGPD has finite moments of all orders for all {\displaystyle \sigma >0} and {\displaystyle -\infty <\xi <\infty }.
The moment-generating function of {\displaystyle Y\sim \mathrm {exGPD} (\sigma ,\xi )} is {\displaystyle M_{Y}(s)=\operatorname {E} \left[e^{sY}\right]={\begin{cases}-{\frac {1}{\xi }}\left(-{\frac {\sigma }{\xi }}\right)^{s}B(s{+}1,,円-1/\xi ),&{\text{for }}&-1<s<\infty ,&\xi <0,\\[1ex]{\frac {1}{\xi }}\left({\frac {\sigma }{\xi }}\right)^{s}B(s{+}1,,1円/\xi -s)&{\text{for }}&-1<s<1/\xi ,&\xi >0,\\[1ex]\sigma ^{s}\Gamma (1+s),&{\text{for }}&-1<s<\infty ,&\xi =0,\end{cases}}} where {\displaystyle B(a,b)} and {\displaystyle \Gamma (a)} denote the beta function and gamma function, respectively.
The expected value of {\displaystyle Y\sim \mathrm {exGPD} (\sigma ,\xi )} depends on the scale {\displaystyle \sigma } and shape {\displaystyle \xi } parameters, while the {\displaystyle \xi } participates through the digamma function: {\displaystyle \operatorname {E} [Y]={\begin{cases}\log \left(-{\frac {\sigma }{\xi }}\right)+\psi (1)-\psi (-1/\xi +1)&{\text{for }}\xi <0,\\[1ex]\log \sigma -\log \xi +\psi (1)-\psi (1/\xi )&{\text{for }}\xi >0,\\[1ex]\log \sigma +\psi (1)&{\text{for }}\xi =0.\end{cases}}} Note that for a fixed value for the {\displaystyle \xi \in (-\infty ,\infty )}, the {\displaystyle \log \ \sigma } plays as the location parameter under the exponentiated generalized Pareto distribution.
The variance of {\displaystyle Y\sim \mathrm {exGPD} (\sigma ,\xi )} depends on the shape parameter {\displaystyle \xi } only through the polygamma function of order 1 (also called the trigamma function): {\displaystyle \operatorname {Var} [Y]={\begin{cases}\psi '(1)-\psi '(-1/\xi +1)&{\text{for }}\xi <0,\\\psi '(1)+\psi '(1/\xi )&{\text{for }}\xi >0,\\\psi '(1)&{\text{for }}\xi =0.\end{cases}}} See the right panel for the variance as a function of {\displaystyle \xi }. Note that {\displaystyle \psi '(1)=\pi ^{2}/6\approx 1.644934}.
Note that the roles of the scale parameter {\displaystyle \sigma } and the shape parameter {\displaystyle \xi } under {\displaystyle Y\sim \mathrm {exGPD} (\sigma ,\xi )} are separably interpretable, which may lead to a robust efficient estimation for the {\displaystyle \xi } than using the {\displaystyle X\sim \mathrm {GPD} (\sigma ,\xi )} [3]. The roles of the two parameters are associated each other under {\displaystyle X\sim \mathrm {GPD} (\mu =0,\sigma ,\xi )} (at least up to the second central moment); see the formula of variance {\displaystyle Var(X)} wherein both parameters are participated.
The Hill's estimator
[edit ]Assume that {\displaystyle X_{1:n}=(X_{1},\cdots ,X_{n})} are {\displaystyle n} observations (need not be i.i.d.) from an unknown heavy-tailed distribution {\displaystyle F} such that its tail distribution is regularly varying with the tail-index {\displaystyle 1/\xi } (hence, the corresponding shape parameter is {\displaystyle \xi }). To be specific, the tail distribution is described as {\displaystyle {\bar {F}}(x)=1-F(x)=L(x)\cdot x^{-1/\xi },,円,円,円,円,円{\text{for some }}\xi >0,,円,円{\text{where }}L{\text{ is a slowly varying function.}}} It is of a particular interest in the extreme value theory to estimate the shape parameter {\displaystyle \xi }, especially when {\displaystyle \xi } is positive (so called the heavy-tailed distribution).
Let {\displaystyle F_{u}} be their conditional excess distribution function. Pickands–Balkema–de Haan theorem (Pickands, 1975; Balkema and de Haan, 1974) states that for a large class of underlying distribution functions {\displaystyle F}, and large {\displaystyle u}, {\displaystyle F_{u}} is well approximated by the generalized Pareto distribution (GPD), which motivated Peak Over Threshold (POT) methods to estimate {\displaystyle \xi }: the GPD plays the key role in POT approach.
A renowned estimator using the POT methodology is the Hill's estimator. Technical formulation of the Hill's estimator is as follows. For {\displaystyle 1\leq i\leq n}, write {\displaystyle X_{(i)}} for the {\displaystyle i}-th largest value of {\displaystyle X_{1},\cdots ,X_{n}}. Then, with this notation, the Hill's estimator (see page 190 of Reference 5 by Embrechts et al [4]) based on the {\displaystyle k} upper order statistics is defined as {\displaystyle {\widehat {\xi }}_{k}^{\text{Hill}}={\widehat {\xi }}_{k}^{\text{Hill}}(X_{1:n})={\frac {1}{k-1}}\sum _{j=1}^{k-1}\log {\bigg (}{\frac {X_{(j)}}{X_{(k)}}}{\bigg )},,円,円,円,円,円,円,円,円{\text{for }}2\leq k\leq n.} In practice, the Hill estimator is used as follows. First, calculate the estimator {\displaystyle {\widehat {\xi }}_{k}^{\text{Hill}}} at each integer {\displaystyle k\in \{2,\cdots ,n\}}, and then plot the ordered pairs {\displaystyle \{(k,{\widehat {\xi }}_{k}^{\text{Hill}})\}_{k=2}^{n}}. Then, select from the set of Hill estimators {\displaystyle \{{\widehat {\xi }}_{k}^{\text{Hill}}\}_{k=2}^{n}} which are roughly constant with respect to {\displaystyle k}: these stable values are regarded as reasonable estimates for the shape parameter {\displaystyle \xi }. If {\displaystyle X_{1},\cdots ,X_{n}} are i.i.d., then the Hill's estimator is a consistent estimator for the shape parameter {\displaystyle \xi } [5].
Note that the Hill estimator {\displaystyle {\widehat {\xi }}_{k}^{\text{Hill}}} makes a use of the log-transformation for the observations {\displaystyle X_{1:n}=(X_{1},\cdots ,X_{n})}. (The Pickand's estimator {\displaystyle {\widehat {\xi }}_{k}^{\text{Pickand}}} also employed the log-transformation, but in a slightly different way [6].)
See also
[edit ]- Burr distribution
- Pareto distribution
- Generalized extreme value distribution
- Exponentiated generalized Pareto distribution
- Pickands–Balkema–de Haan theorem
References
[edit ]- ^ a b Norton, Matthew; Khokhlov, Valentyn; Uryasev, Stan (2019). "Calculating CVaR and bPOE for common probability distributions with application to portfolio optimization and density estimation" (PDF). Annals of Operations Research. 299 (1–2). Springer: 1281–1315. arXiv:1811.11301 . doi:10.1007/s10479-019-03373-1. S2CID 254231768. Archived from the original (PDF) on 2023年03月31日. Retrieved 2023年02月27日.
- ^ Coles, Stuart (2001年12月12日). An Introduction to Statistical Modeling of Extreme Values. Springer. p. 75. ISBN 9781852334598.
- ^ Dargahi-Noubary, G. R. (1989). "On tail estimation: An improved method". Mathematical Geology. 21 (8): 829–842. Bibcode:1989MatGe..21..829D. doi:10.1007/BF00894450. S2CID 122710961.
- ^ Hosking, J. R. M.; Wallis, J. R. (1987). "Parameter and Quantile Estimation for the Generalized Pareto Distribution". Technometrics. 29 (3): 339–349. doi:10.2307/1269343. JSTOR 1269343.
- ^ Davison, A. C. (1984年09月30日). "Modelling Excesses over High Thresholds, with an Application". In de Oliveira, J. Tiago (ed.). Statistical Extremes and Applications. Kluwer. p. 462. ISBN 9789027718044.
- ^ Embrechts, Paul; Klüppelberg, Claudia; Mikosch, Thomas (1997年01月01日). Modelling extremal events for insurance and finance. Springer. p. 162. ISBN 9783540609315.
- ^ Castillo, Enrique, and Ali S. Hadi. "Fitting the generalized Pareto distribution to data." Journal of the American Statistical Association 92.440 (1997): 1609-1620.
- ^ a b Jewson, Stephen; Sweeting, Trevor; Jewson, Lynne (2025年02月20日). "Reducing reliability bias in assessments of extreme weather risk using calibrating priors". Advances in Statistical Climatology, Meteorology and Oceanography. 11 (1): 1–22. Bibcode:2025ASCMO..11....1J. doi:10.5194/ascmo-11-1-2025 . ISSN 2364-3579.
Further reading
[edit ]- Pickands, James (1975). "Statistical inference using extreme order statistics" (PDF). Annals of Statistics. 3 s: 119–131. doi:10.1214/aos/1176343003 .
- Balkema, A.; De Haan, Laurens (1974). "Residual life time at great age". Annals of Probability. 2 (5): 792–804. doi:10.1214/aop/1176996548 .
- Lee, Seyoon; Kim, J.H.K. (2018). "Exponentiated generalized Pareto distribution:Properties and applications towards extreme value theory". Communications in Statistics - Theory and Methods. 48 (8): 1–25. arXiv:1708.01686 . doi:10.1080/03610926.2018.1441418. S2CID 88514574.
- N. L. Johnson; S. Kotz; N. Balakrishnan (1994). Continuous Univariate Distributions Volume 1, second edition. New York: Wiley. ISBN 978-0-471-58495-7. Chapter 20, Section 12: Generalized Pareto Distributions.
- Barry C. Arnold (2011). "Chapter 7: Pareto and Generalized Pareto Distributions". In Duangkamon Chotikapanich (ed.). Modeling Distributions and Lorenz Curves. New York: Springer. ISBN 9780387727967.
- Arnold, B. C.; Laguna, L. (1977). On generalized Pareto distributions with applications to income data. Ames, Iowa: Iowa State University, Department of Economics.