Pwcor computes the pooled-within classes correlation between two dichotomous
variables for all groups, regardless of whether they are heterogeneous or
homogeneous with respect to X or Y (Cronbach, 1976), and for those groups
that are heterogeneous only with respect to X or Y (Cahan, 1986).
Silver, N.C., & Hittner, J.B. (1998). PWCOR: A Microsoft FORTRAN 77 program
for computing the "pooled-within classes" correlation. Applied Psychological
Measurement, 22, 52.