Pearson introduced two new methods in 1900: the tetrachoric (i.e. “four-fold”) correlation coefficient (rt); and his phi coefficient (φ), known later as “Pearson’s phi coefficient” for discrete variables. Both methods measure the association between two variables, designed for 2 × 2 (or four-fold) tables, which can be placed into two mutually exclusive categories, (called “dichotomous” variables).
