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Noncentral chi-squared distribution : ウィキペディア英語版
Noncentral chi-squared distribution
\left( \sqrt, \sqrt \right) with Marcum Q-function Q_M(a,b)
|
mean =k+\lambda\,|
median =|
mode =|
variance =2(k+2\lambda)\,|
skewness =\frac|
entropy =|
mgf =\frac\right)}2t<1|
char =\frac\right)}}
In probability theory and statistics, the noncentral chi-squared or noncentral \chi^2 distribution is a generalization of the chi-squared distribution. This distribution often arises in the power analysis of statistical tests in which the null distribution is (perhaps asymptotically) a chi-squared distribution; important examples of such tests are the likelihood ratio tests.
==Background==
Let (X_1, X_2, \ldots, X_i, \ldots, X_k) be ''k'' independent, normally distributed random variables with means \mu_i and unit variances. Then the random variable
: \sum_^k X_i^2
is distributed according to the noncentral chi-squared distribution. It has two parameters: k which specifies the number of degrees of freedom (i.e. the number of X_i), and \lambda which is related to the mean of the random variables X_i by:
: \lambda=\sum_^k \mu_i^2.
\lambda is sometimes called the noncentrality parameter. Note that some references define \lambda in other ways, such as half of the above sum, or its square root.
This distribution arises in multivariate statistics as a derivative of the multivariate normal distribution. While the central chi-squared distribution is the squared norm of a random vector with N(0_k,I_k) distribution (i.e., the squared distance from the origin of a point taken at random from that distribution), the non-central \chi^2 is the squared norm of a random vector with N(\mu,I_k) distribution. Here 0_k is a zero vector of length ''k'', \mu = (\mu_1, \ldots, \mu_k) and I_k is the identity matrix of size ''k''.

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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