Summary of the steps in constructing the multinomial regression model using the backward elimination method.

aThe χ2 for removal is based on the likelihood ratio test.bBecause each proband’s proportions of older brothers, older sisters, younger brothers, and younger sisters is necessarily summed to 1.00, these proportions were perfectly multicollinear. To reduce the multicollinearity, the computational algorithm of the SPSS multinomial logistic regression program eliminated the proportion of younger brothers from the set of predictor variables.