Noncentral chi distribution
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degrees of freedom
In probability theory and statistics, the noncentral chi distribution is a generalization of the chi distribution. If are k independent, normally distributed random variables with means and variances , then the statistic
is distributed according to the noncentral chi distribution. The noncentral chi distribution has two parameters: which specifies the number of degrees of freedom (i.e. the number of ), and which is related to the mean of the random variables by:
Probability density function
The probability density function (pdf) is
where is a modified Bessel function of the first kind.
The first few raw moments are:
The pdf of the noncentral chi distribution is a solution to the following differential equation:
Bivariate non-central chi distribution
with positive definite. Define
Then the joint distribution of U, V is central or noncentral bivariate chi distribution with n degrees of freedom. If either or both or the distribution is a noncentral bivariate chi distribution.
- If is a random variable with the non-central chi distribution, the random variable will have the noncentral chi-squared distribution. Other related distributions may be seen there.
- If is chi distributed: then is also non-central chi distributed: . In other words, the chi distribution is a special case of the non-central chi distribution (i.e., with a non-centrality parameter of zero).
- A noncentral chi distribution with 2 degrees of freedom is equivalent to a Rice distribution with .
- If X follows a noncentral chi distribution with 1 degree of freedom and noncentrality parameter λ, then σX follows a folded normal distribution whose parameters are equal to σλ and σ2 for any value of σ.