Fitting multiple threshold models to two-way twin pair contingency tables for AD symptom count confirmed that the observed data met the assumption of an underlying normal liability distribution in like-sex male (Chi-square=29.24, d.f.=30, p=0.51) and female (Chi-square=32.61, d.f.=30, p=0.34) twin pairs, and for the 5-group analysis including unlike-sex pairs (Chi-square=75.05, d.f.=75, p=0.48).
