# Noncentral F-distribution

In probability theory and statistics, the noncentral F-distribution is a continuous probability distribution that is a generalization of the (ordinary) F-distribution. It describes the distribution of the quotient (X/n1)/(Y/n2), where the numerator X has a noncentral chi-squared distribution with n1 degrees of freedom and the denominator Y has a central chi-squared distribution with n2 degrees of freedom. It is also required that X and Y are statistically independent of each other.

It is the distribution of the test statistic in analysis of variance problems when the null hypothesis is false. The noncentral F-distribution is used to find the power function of such a test.

## Occurrence and specification

If $X$  is a noncentral chi-squared random variable with noncentrality parameter $\lambda$  and $\nu _{1}$  degrees of freedom, and $Y$  is a chi-squared random variable with $\nu _{2}$  degrees of freedom that is statistically independent of $X$ , then

$F={\frac {X/\nu _{1}}{Y/\nu _{2}}}$

is a noncentral F-distributed random variable. The probability density function (pdf) for the noncentral F-distribution is

$p(f)=\sum \limits _{k=0}^{\infty }{\frac {e^{-\lambda /2}(\lambda /2)^{k}}{B\left({\frac {\nu _{2}}{2}},{\frac {\nu _{1}}{2}}+k\right)k!}}\left({\frac {\nu _{1}}{\nu _{2}}}\right)^{{\frac {\nu _{1}}{2}}+k}\left({\frac {\nu _{2}}{\nu _{2}+\nu _{1}f}}\right)^{{\frac {\nu _{1}+\nu _{2}}{2}}+k}f^{\nu _{1}/2-1+k}$

when $f\geq 0$  and zero otherwise. The degrees of freedom $\nu _{1}$  and $\nu _{2}$  are positive. The term $B(x,y)$  is the beta function, where

$B(x,y)={\frac {\Gamma (x)\Gamma (y)}{\Gamma (x+y)}}.$

The cumulative distribution function for the noncentral F-distribution is

$F(x|d_{1},d_{2},\lambda )=\sum \limits _{j=0}^{\infty }\left({\frac {\left({\frac {1}{2}}\lambda \right)^{j}}{j!}}e^{-{\frac {\lambda }{2}}}\right)I\left({\frac {d_{1}x}{d_{2}+d_{1}x}}{\bigg |}{\frac {d_{1}}{2}}+j,{\frac {d_{2}}{2}}\right)$

where $I$  is the regularized incomplete beta function.

The mean and variance of the noncentral F-distribution are

$\operatorname {E} \left[F\right]={\begin{cases}{\frac {\nu _{2}(\nu _{1}+\lambda )}{\nu _{1}(\nu _{2}-2)}}&\nu _{2}>2\\{\text{Does not exist}}&\nu _{2}\leq 2\\\end{cases}}$

and

$\operatorname {Var} \left[F\right]={\begin{cases}2{\frac {(\nu _{1}+\lambda )^{2}+(\nu _{1}+2\lambda )(\nu _{2}-2)}{(\nu _{2}-2)^{2}(\nu _{2}-4)}}\left({\frac {\nu _{2}}{\nu _{1}}}\right)^{2}&\nu _{2}>4\\{\text{Does not exist}}&\nu _{2}\leq 4.\\\end{cases}}$

## Special cases

When λ = 0, the noncentral F-distribution becomes the F-distribution.

## Related distributions

$Z=\lim _{\nu _{2}\to \infty }\nu _{1}F$

where F has a noncentral F-distribution.