Documentation

chi2inv

Chi-square inverse cumulative distribution function

Syntax

X = chi2inv(P,V)

Description

X = chi2inv(P,V) computes the inverse of the chi-square cdf with degrees of freedom specified by V for the corresponding probabilities in P. P and V can be vectors, matrices, or multidimensional arrays that have the same size. A scalar input is expanded to a constant array with the same dimensions as the other inputs.

The degrees of freedom parameters in V must be positive integers, and the values in P must lie in the interval [0 1].

The inverse chi-square cdf for a given probability p and ν degrees of freedom is

x=F1(p|ν)={x:F(x|ν)=p}

where

p=F(x|ν)=0xt(ν2)/2et/22ν/2Γ(ν/2)dt

and Γ( · ) is the Gamma function. Each element of output X is the value whose cumulative probability under the chi-square cdf defined by the corresponding degrees of freedom parameter in V is specified by the corresponding value in P.

Examples

Find a value that exceeds 95% of the samples from a chi-square distribution with 10 degrees of freedom.

x = chi2inv(0.95,10)
x =
  18.3070

You would observe values greater than 18.3 only 5% of the time by chance.

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