Drasgow, F. (1986), “Polychoric and Polyserial Correlations,” in S. Kotz and N. L. Johnson, eds., Encyclopedia of Statistical Sciences, volume 7, 68–74, New York: John Wiley & Sons.
Olsson, U. (1979), “Maximum Likelihood Estimation of the Polychoric Correlation Coefficient,” Psychometrika, 12, 443–460.