(kī) A quantity distributed as
where are independent, Gaussian, with zero mean, and with unit variance. The chi-square test is a statistical test of data distribution. If fi are observed, frequencies of occurrence and ei are the expected number of occurrences, the goodness of fit y is
The goodness of fit can be related (by the use of tables) to the probability that the observed data fit the model with the assumed number of degrees of freedom.