Dictionary:Chi-square

(kī) A quantity distributed as

$\chi _{1}^{2}+\chi _{2}^{2}+\dots +\chi _{k}^{2}$,

where $\chi _{1},\chi _{2},\dots ,\chi _{k}$ are independent, Gaussian, with zero mean, and with unit variance. The chi-square test is a statistical test of data distribution. If f_i are observed, frequencies of occurrence and e_i are the expected number of occurrences, the goodness of fit y is

$y=\sum {\frac {\left(f_{i}-e_{i}\right)^{2}}{e_{i}}}$.

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.