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Gaussian copula : ウィキペディア英語版
Copula (probability theory)
In probability theory and statistics, a copula is a multivariate probability distribution for which the marginal probability distribution of each variable is uniform. Copulas are used to describe the dependence between random variables. Their name comes from the Latin for "link" or "tie", similar but unrelated to grammatical copulas in linguistics.
Sklar's Theorem states that any multivariate joint distribution can be written in terms of univariate marginal distribution functions and a copula which describes the dependence structure between the variables.
Copulas are popular in high-dimensional statistical applications as they allow one to easily model and estimate the distribution of random vectors by estimating marginals and copulae separately. There are many parametric copula families available, which usually have parameters that control the strength of dependence. Some popular parametric copula models are outlined below.
==Mathematical definition==
Consider a random vector (X_1,X_2,\dots,X_d). Suppose its margins are continuous, i.e. the marginal CDFs F_i(x) = \mathbb(x ) are continuous functions. By applying the probability integral transform to each component, the random vector
:(U_1,U_2,\dots,U_d)=\left(F_1(X_1),F_2(X_2),\dots,F_d(X_d)\right)
has uniformly distributed marginals.
The copula of (X_1,X_2,\dots,X_d) is defined as the joint cumulative distribution function of (U_1,U_2,\dots,U_d):
:C(u_1,u_2,\dots,u_d)=\mathbb(u_1,U_2\leq u_2,\dots,U_d\leq u_d ) .
The copula ''C'' contains all information on the dependence structure between the components of (X_1,X_2,\dots,X_d) whereas the marginal cumulative distribution functions F_i contain all information on the marginal distributions.
The importance of the above is that the reverse of these steps can be used to generate pseudo-random samples from general classes of multivariate probability distributions. That is, given a procedure to generate a sample (U_1,U_2,\dots,U_d) from the copula distribution, the required sample can be constructed as
:(X_1,X_2,\dots,X_d) = \left(F_1^(U_1),F_2^(U_2),\dots,F_d^(U_d)\right).
The inverses F_i^ are unproblematic as the F_i were assumed to be continuous. The above formula for the copula function can be rewritten to correspond to this as:
:C(u_1,u_2,\dots,u_d)=\mathbb(F_1^(u_1),X_2\leq F_2^(u_2),\dots,X_d\leq F_d^(u_d) ) .

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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