${\chi }^{2}=\mathrm{\Sigma }\frac{{\left(O-E\right)}^{2}}{E}$

Description:

Math equation. One begins by finding the difference between the observed and expected number of subjects in each cell of the contingency table. The difference is then squared and divided by the expected number for each cell. The results are then summed for all cells in the contingency table and the result is the chi squared statistic.