Statistics/Chi-squared distribution
The probability density function (pdf) of the chi-squared distribution as used in statistics is
- , where
Here, denotes the Gamma_function.
The use of the gamma function in the equation below reflects the chi-squared distribution's origin as a special case of the gamma distribution.
In probability theory and statistics, the gamma distribution is a two-parameter family of continuous probability distributions. The exponential distribution, Erlang distribution, and chi-square distribution are special cases of the gamma distribution.
The probability density function (pdf) of the gamma distribution is given by the formula
where Γ(k) is the Gamma_function, with shape parameter k and a scale parameter θ.
The cumulative probability distribution of the gamma distribution is the area under the curve of the ditribution, which indicates the increasing probability of the x value of a single random point within the gamma distribution being less than or equal to the x value of the cumulative probability distribution. The gamma cumulative probability distribution function can be calculated as
where is the lower incomplete gamma function.
The lower incomplete gamma function can be calculated as
and so, for the chi-squared cumulative probability distribution with chi-squared k, we have, substituting chi-square k into s as k/2,