Noncentral chi-squared distribution

Let ${\displaystyle (X_{1},X_{2},\ldots ,X_{i},\ldots ,X_{k})}$ be k independent, normally distributed random variables with means ${\displaystyle \mu _{i}}$ and unit variances. Then the random variable

${\displaystyle \sum _{i=1}^{k}X_{i}^{2}}$

is distributed according to the noncentral chi-square distribution. It has two parameters: ${\displaystyle k}$ which specifies the number of degrees of freedom (i.e. the number of ${\displaystyle X_{i}}$), and ${\displaystyle \lambda }$ which is related to the mean of the random variables ${\displaystyle X_{i}}$ by:

${\displaystyle \lambda =\sum _{i=1}^{k}\mu _{i}^{2}.}$

${\displaystyle \lambda }$ is sometimes called the noncentrality parameter. Note that some references define ${\displaystyle \lambda }$ 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-square distribution is the squared norm of a random vector with ${\displaystyle N(0_{k},I_{k})}$ distribution (i.e., the squared distance from the origin to a point taken at random from that distribution), the non-central ${\displaystyle \chi ^{2}}$ is the squared norm of a random vector with ${\displaystyle N(\mu ,I_{k})}$ distribution. Here ${\displaystyle 0_{k}}$ is a zero vector of length k, ${\displaystyle \mu =(\mu _{1},\ldots ,\mu _{k})}$ and ${\displaystyle I_{k}}$ is the identity matrix of size k.

The probability density function (pdf) is given by

${\displaystyle f_{X}(x;k,\lambda )=\sum _{i=0}^{\infty }{\frac {e^{-\lambda /2}(\lambda /2)^{i}}{i!}}f_{Y_{k+2i}}(x),}$

where ${\displaystyle Y_{q}}$ is distributed as chi-square with ${\displaystyle q}$ degrees of freedom.

From this representation, the noncentral chi-square distribution is seen to be a Poisson-weighted mixture of central chi-square distributions. Suppose that a random variable J has a Poisson distribution with mean ${\displaystyle \lambda /2}$, and the conditional distribution of Z given J = i is chi-square with k + 2i degrees of freedom. Then the unconditional distribution of Z is non-central chi-square with k degrees of freedom, and non-centrality parameter ${\displaystyle \lambda }$.

Alternatively, the pdf can be written as

${\displaystyle f_{X}(x;k,\lambda )={\frac {1}{2}}e^{-(x+\lambda )/2}\left({\frac {x}{\lambda }}\right)^{k/4-1/2}I_{k/2-1}({\sqrt {\lambda x}})}$

where ${\displaystyle I_{\nu }(y)}$ is a modified Bessel function of the first kind given by

${\displaystyle I_{\nu }(y)=(y/2)^{\nu }\sum _{j=0}^{\infty }{\frac {(y^{2}/4)^{j}}{j!\Gamma (\nu +j+1)}}.}$

Using the relation between Bessel functions and hypergeometric functions, the pdf can also be written as:[1]

${\displaystyle f_{X}(x;k,\lambda )={{\rm {e}}^{-\lambda /2}}_{0}F_{1}(;k/2;\lambda x/4){\frac {1}{2^{k/2}\Gamma (k/2)}}{\rm {e}}^{-x/2}x^{k/2-1}.}$

Siegel (1979) discusses the case k = 0 specifically (zero degrees of freedom), in which case the distribution has a discrete component at zero.

The derivation of the probability density function is most easily done by performing the following steps:

1. Since ${\displaystyle X_{1},\ldots ,X_{k}}$ have unit variances, their joint distribution is spherically symmetric, up to a location shift.
2. The spherical symmetry then implies that the distribution of ${\displaystyle X=X_{1}^{2}+\cdots +X_{k}^{2}}$ depends on the means only through the squared length, ${\displaystyle \lambda =\mu _{1}^{2}+\cdots +\mu _{k}^{2}}$. Without loss of generality, we can therefore take ${\displaystyle \mu _{1}={\sqrt {\lambda }}}$ and ${\displaystyle \mu _{2}=\cdots =\mu _{k}=0}$.
3. Now derive the density of ${\displaystyle X=X_{1}^{2}}$ (i.e. the k = 1 case). Simple transformation of random variables shows that

${\displaystyle {\begin{aligned}f_{X}(x,1,\lambda )&={\frac {1}{2{\sqrt {x}}}}\left(\phi ({\sqrt {x}}-{\sqrt {\lambda }})+\phi ({\sqrt {x}}+{\sqrt {\lambda }})\right)\\&={\frac {1}{\sqrt {2\pi x}}}e^{-(x+\lambda )/2}\cosh({\sqrt {\lambda x}}),\end{aligned}}}$

where ${\displaystyle \phi (\cdot )}$ is the standard normal density.

1. Expand the cosh term in a Taylor series. This gives the Poisson-weighted mixture representation of the density, still for k = 1. The indices on the chi-square random variables in the series above are 1 + 2i in this case.
2. Finally, for the general case. We've assumed, without loss of generality, that ${\displaystyle X_{2},\ldots ,X_{k}}$ are standard normal, and so ${\displaystyle X_{2}^{2}+\cdots +X_{k}^{2}}$ has a central chi-square distribution with (k − 1) degrees of freedom, independent of ${\displaystyle X_{1}^{2}}$. Using the poisson-weighted mixture representation for ${\displaystyle X_{1}^{2}}$, and the fact that the sum of chi-square random variables is also a chi-square, completes the result. The indices in the series are (1 + 2i) + (k − 1) = k + 2i as required.

• If ${\displaystyle V}$ is chi-square distributed ${\displaystyle V\sim \chi _{k}^{2}}$ then ${\displaystyle V}$ is also non-central chi-square distributed: ${\displaystyle V\sim {\chi '}_{k}^{2}(0)}$
• A linear combination of independent noncentral chi-squared variables ${\displaystyle \xi =\sum _{i}\lambda _{i}Y_{i}+c,\quad Y_{i}\sim \chi '^{2}(m_{i},\delta _{i}^{2})}$, is generalized chi-square distributed.
• If ${\displaystyle V_{1}\sim {\chi '}_{k_{1}}^{2}(\lambda )}$ and ${\displaystyle V_{2}\sim {\chi '}_{k_{2}}^{2}(0)}$ and ${\displaystyle V_{1}}$ is independent of ${\displaystyle V_{2}}$ then a noncentral F-distributed variable is developed as ${\displaystyle {\frac {V_{1}/k_{1}}{V_{2}/k_{2}}}\sim F'_{k_{1},k_{2}}(\lambda )}$
• If ${\displaystyle J\sim \mathrm {Poisson} \left({{\frac {1}{2}}\lambda }\right)}$, then ${\displaystyle \chi _{k+2J}^{2}\sim {\chi '}_{k}^{2}(\lambda )}$
• If ${\displaystyle V\sim {\chi '}_{2}^{2}(\lambda )}$, then ${\displaystyle {\sqrt {V}}}$ takes the Rice distribution with parameter ${\displaystyle {\sqrt {\lambda }}}$.
• Normal approximation:[7] if ${\displaystyle V\sim {\chi '}_{k}^{2}(\lambda )}$, then ${\displaystyle {\frac {V-(k+\lambda )}{\sqrt {2(k+2\lambda )}}}\to N(0,1)}$ in distribution as either ${\displaystyle k\to \infty }$ or ${\displaystyle \lambda \to \infty }$.
• If ${\displaystyle V_{1}\sim {\chi '}_{k_{1}}^{2}(\lambda _{1})}$and ${\displaystyle V_{2}\sim {\chi '}_{k_{2}}^{2}(\lambda _{2})}$, where ${\displaystyle V_{1},V_{2}}$ are independent, then ${\displaystyle W=(V_{1}+V_{2})\sim {\chi '}_{k}^{2}(\lambda _{1}+\lambda _{2})}$ where ${\displaystyle k=k_{1}+k_{2}}$.
• In general, for a finite set of ${\displaystyle V_{i}\sim {\chi '}_{k_{i}}^{2}(\lambda _{i}),i\in \left\{1..N\right\}}$, the sum of these non-central chi-square distributed random variables ${\displaystyle Y=\sum _{i=1}^{N}V_{i}}$has the distribution ${\displaystyle Y\sim {\chi '}_{k_{y}}^{2}(\lambda _{y})}$where ${\displaystyle k_{y}=\sum _{i=1}^{N}k_{i},\lambda _{y}=\sum _{i=1}^{N}\lambda _{i}}$. This can be seen using moment generating functions as follows: ${\displaystyle M_{Y}(t)=M_{\sum _{i=1}^{N}V_{i}}(t)=\prod _{i=1}^{N}M_{V_{i}}(t)}$by the independence of the ${\displaystyle V_{i}}$random variables. It remains to plug in the MGF for the non-central chi square distributions into the product and compute the new MGF - this is left as an exercise. Alternatively it can be seen via the interpretation in the background section above as sums of squares of independent normally distributed random variables with variances of 1 and the specified means.
• The Complex noncentral Chi squared distribution has applications in radio communication and radar systems. Let ${\displaystyle (z_{1},\ldots ,z_{k})}$ be independent scalar complex random variables with circular symmetry, means of ${\displaystyle \mu _{i}}$ and unit variances: ${\displaystyle \operatorname {E} \left|z_{i}-\mu _{i}\right|^{2}=1}$ . Then the real random variable ${\displaystyle S=\sum _{i=1}^{k}\left|z_{i}\right|^{2}}$ is distributed according to the complex noncentral chi-square distribution

${\displaystyle f_{S}(S)=\left({\frac {S}{\lambda }}\right)^{(k-1)/2}e^{-(S+\lambda )}I_{k-1}(2{\sqrt {S\lambda }})}$

where ${\displaystyle \lambda =\sum _{i=1}^{k}\left|\mu _{i}\right|^{2}.}$

Transformations

Sankaran (1963) discusses the transformations of the form ${\displaystyle z=[(X-b)/(k+\lambda )]^{1/2}}$. He analyzes the expansions of the cumulants of ${\displaystyle z}$ up to the term ${\displaystyle O((k+\lambda )^{-4})}$ and shows that the following choices of ${\displaystyle b}$ produce reasonable results:

• ${\displaystyle b=(k-1)/2}$ makes the second cumulant of ${\displaystyle z}$ approximately independent of ${\displaystyle \lambda }$
• ${\displaystyle b=(k-1)/3}$ makes the third cumulant of ${\displaystyle z}$ approximately independent of ${\displaystyle \lambda }$
• ${\displaystyle b=(k-1)/4}$ makes the fourth cumulant of ${\displaystyle z}$ approximately independent of ${\displaystyle \lambda }$

Also, a simpler transformation ${\displaystyle z_{1}=(X-(k-1)/2)^{1/2}}$ can be used as a variance stabilizing transformation that produces a random variable with mean ${\displaystyle (\lambda +(k-1)/2)^{1/2}}$ and variance ${\displaystyle O((k+\lambda )^{-2})}$.

Usability of these transformations may be hampered by the need to take the square roots of negative numbers.

Various chi and chi-square distributions

Name	Statistic
chi-square distribution	${\displaystyle \sum _{1}^{k}\left({\frac {X_{i}-\mu _{i}}{\sigma _{i}}}\right)^{2}}$
noncentral chi-square distribution	${\displaystyle \sum _{1}^{k}\left({\frac {X_{i}}{\sigma _{i}}}\right)^{2}}$
chi distribution	${\displaystyle {\sqrt {\sum _{1}^{k}\left({\frac {X_{i}-\mu _{i}}{\sigma _{i}}}\right)^{2}}}}$
noncentral chi distribution	${\displaystyle {\sqrt {\sum _{1}^{k}\left({\frac {X_{i}}{\sigma _{i}}}\right)^{2}}}}$

• Abramowitz, M. and Stegun, I. A. (1972), Handbook of Mathematical Functions, Dover. Section 26.4.25.
• Johnson, N. L., Kotz, S., Balakrishnan, N. (1995), Continuous Univariate Distributions, Volume 2 (2nd Edition), Wiley. ISBN 0-471-58494-0
• Muirhead, R. (2005) Aspects of Multivariate Statistical Theory (2nd Edition). Wiley. ISBN 0-471-76985-1
• Siegel, A. F. (1979), "The noncentral chi-squared distribution with zero degrees of freedom and testing for uniformity", Biometrika, 66, 381–386
• Press, S.J. (1966), "Linear combinations of non-central chi-squared variates", The Annals of Mathematical Statistics, 37 (2): 480–487, doi:10.1214/aoms/1177699531, JSTOR 2238621