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Generalized Dirichlet distribution : ウィキペディア英語版
Generalized Dirichlet distribution
In statistics, the generalized Dirichlet distribution (GD) is a generalization of the Dirichlet distribution with a more general covariance structure and almost twice the number of parameters. Random variables with a GD distribution are neutral but not completely neutral.〔R. J. Connor and J. E. Mosiman 1969. ''Concepts of independence for proportions with a generalization of the Dirichlet distribution''. Journal of the American Statistical Association, volume 64, pp194--206〕
The density function of p_1,\ldots,p_ is
:
\left()^
p_k^
\prod_^\left()

where we define p_k= 1- \sum_^p_i. Here B(x,y) denotes the Beta function. This reduces to the standard Dirichlet distribution if b_=a_i+b_i for 2\leqslant i\leqslant k-1 (b_0 is arbitrary).
For example, if ''k=4'', then the density function of p_1,p_2,p_3 is
:
\left()^
p_1^p_2^p_3^p_4^\left(p_2+p_3+p_4\right)^\left(p_3+p_4\right)^

where p_1+p_2+p_3<1 and p_4=1-p_1-p_2-p_3.
Connor and Mosimann define the PDF as they did for the following reason. Define random variables z_1,\ldots,z_ with z_1=p_1, z_2=p_2/\left(1-p_1\right), z_3=p_3/\left(1-(p_1+p_2)\right),\ldots,z_i = p_i/\left(1-p_1+\cdots+p_\right). Then p_1,\ldots,p_k have the generalized Dirichlet distribution as parametrized above, if the z_i are iid beta with parameters a_i,b_i, i=1,\ldots,k-1.
== Alternative form given by Wong ==
Wong 〔T.-T. Wong 1998. ''Generalized Dirichlet distribution in Bayesian analysis''. Applied Mathematics and Computation, volume 97, pp165-181〕 gives the slightly more concise form for x_1+\cdots +x_k\leqslant 1
:
\prod_^k\frac}

where \gamma_j=\beta_j-\alpha_-\beta_ for 1\leqslant j\leqslant k-1 and \gamma_k=\beta_k-1. Note that Wong defines a distribution over a k dimensional space (implicitly defining x_=1-\sum_^kx_i) while Connor and Mosiman use a k-1 dimensional space with x_k=1-\sum_^x_i.
==General moment function==

If X=\left(X_1,\ldots,X_k\right)\sim GD_k\left(\alpha_1,\ldots,\alpha_k;\beta_1,\ldots,\beta_k\right), then
:
E\left(X_k^\right )=
\prod_^k
\frac

where \delta_j=r_+r_+\cdots +r_k for j=1,2,\cdots,k-1 and \delta_k=0. Thus
:
E\left(X_j\right)=\frac\prod_^\frac.


抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
ウィキペディアで「Generalized Dirichlet distribution」の詳細全文を読む



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