Wilks' lambda distribution

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In statistics, Wilks' lambda distribution (named for Samuel S. Wilks), is a probability distribution used in multivariate hypothesis testing, especially with regard to the likelihood-ratio test. It is a generalization of the F-distribution, and generalizes Hotelling's T-square distribution in the same way that the F-distribution generalizes Student's t-distribution.

Wilks' lambda distribution is related to two independent Wishart distributed variables, and is defined as follows,[1]

given

A \sim W_p(I, m) \qquad B \sim W_p(I, n)

independent and with m \ge p

\lambda = \frac{|A|}{|A+B|} = \frac{1}{|I+A^{-1}B|} \sim \Lambda(p,m,n).

The distribution can be related to a product of independent Beta distributed random variables

u_i \sim B\left(\frac{m+i-p}{2},\frac{p}{2}\right)
\prod_{i=1}^n u_i \sim \Lambda(p,m,n).

In the context of likelihood-ratio tests m is typically the error degrees of freedom, and n is the hypothesis degrees of freedom, so that n + m is the total degrees of freedom.[1]

For large m Bartlett's approximation [2] allows Wilks' lambda to be approximated with a Chi-square distribution

\left(\frac{p-n+1}{2}-m\right)\log \Lambda(p,m,n) \sim \chi^2_{np}.[1]

[edit] References

  1. ^ a b c Mardia, K.V.; J.T. Kent, J.M. Bibby (1979). Multivariate Analysis. Academic Press. 
  2. ^ Bartlett, M.S. (1954). "A note on multiplying factors for various χ2 approximations". J. Royal Statist. Soc. Series B 16: 296–298. 
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