Phi coefficient
In statistics, the phi coefficient (also referred to as the "mean square contingency coefficient" and denoted by φ (or rφ)) is a measure of association for two binary variables. Introduced by Karl Pearson, this measure is similar to the Pearson correlation coefficient in its interpretation. In fact, a Pearson correlation coefficient estimated for two binary variables will return the phi coefficient. The square of the Phi coefficient is related to the chi-squared statistic for a 2×2 contingency table (see Pearson's chi-squared test)