Chi-squared distribution

A sum of squared independent standard normal variables has a chi-squared distribution.

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Gamma and beta distributions
Gamma distributions model positive quantities; beta distributions model values on the unit interval.
Student's t distribution
A standard normal divided by an independent scaled chi-squared root gives a t distribution.
Analysis of variance
ANOVA partitions variation to test mean differences under model assumptions.
Autocorrelation
Autocorrelation measures dependence between a process and its lagged values.
Bayes theorem
Bayes theorem relates conditional probabilities in opposite directions using prior probabilities and a nonzero evidence probability.