Michael V. Scovetta
Knowledge Base: Generalized Extreme Value Distribution
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In probability theory and statistics, the generalized extreme value distribution (GEV) is a family of continuous probability distributions developed within extreme value theory to combine the Gumbel, Fréchet and Weibull families also known as type I, II and III extreme value distributions. By the extreme value theorem the GEV distribution is the limit distribution of properly normalized maxima of a sequence of independent and identically distributed random variables. Because of this, the GEV distribution is used as an approximation to model the maxima of long (finite) sequences of random variables.
In some fields of application the generalized extreme value distribution is a known as the Fisher–Tippett distribution, named after Sir Ronald Aylmer Fisher (1890-1962) and Leonard Henry Caleb Tippett (1902-1985) who recognised three function forms outlined below. However usage of this name is sometimes restricted to mean the special case of the Gumbel distribution.
The generalized extreme value distribution has cumulative distribution function
for , where is the location parameter, the scale parameter and the shape parameter.
The density function is, consequently
again, for .
Mean, standard deviation, mode, skewness and kurtosis excess
The skewness is
The excess kurtosis is:
where , k=1,2,3,4, and is Gamma function.
Link to Fréchet, Weibull and Gumbel families
The shape parameter governs the tail behaviour of the distribution. The sub-families defined by , and correspond, respectively, to the Gumbel, Fréchet and Weibull families, whose cumulative distribution functions are displayed below.
where and .
Remark I: The theory here relates to maxima and the distribution being discussed is an extreme value distribution for maxima. A Generalised Extreme Value distribution for minima can be obtained, for example by substituting (-x) for x in the distribution function and this yields a separate family of distributions.
Remark II: The ordinary Weibull distribution arises in reliability applications and is obtained from the distribution here by using the variable , which gives a strictly positive support - in contrast to the use in the extreme value theory here. This arises because the Weibull distribution is used in cases that deal with the minimum rather than the maximum. The distribution here has an addition parameter compared to the usual form of the Weibull distribution and, in addition, is reversed so that the distribution has an upper bound rather than a lower bound. Importantly, in applications of the GEV, the upper bound is unknown and so must be estimated while when applying the Weibull distribution the lower bound is known to be zero.
Remark III: Note the differences in the ranges of interest for the three extreme value distributions: Gumbel is unlimited, Fréchet has a lower limit, while the reversed Weibull has an upper limit.
One can link the type I to types II and III the following way: if the cumulative distribution function of some random variable is of type II: , then the cumulative distribution function of is of type I, namely . Similarly, if the cumulative distribution function of is of type III: , the cumulative distribution function of is of type I: .
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