PsiChiSquare (df) is a distribution with a finite lower bound of zero, and an infinite upper bound. It is usually used in statistical significance tests.
The Chi Square distribution is a special case of the Gamma distribution. A Chi Square random variable with parameter df = 2 is the same as an Exponential random variable with mean 0.5. As the parameter df approaches infinity, the Chi Square distribution tends to a Normal distribution.
Alternate Formulation: PsiChiSquareAlt
PsiChiSquareAlt is the PsiChiSquare distribution defined through alternative arguments. One parameter is required which must be chosen from the following list: percentile or var.