We used several complementary approaches to examine the genetic contributions to the risk of AD. We first analyzed the dichotomous phenotype of AD, because it is the most widely studied phenotype and most directly reflects the clinically relevant trait. We then examined each of the DSM-IV criteria for dependence individually, as one approach to reduce heterogeneity. We also employed latent class analysis to identify subgroups of individuals defined on the basis of their pattern of endorsement of DSM-IV criteria and used the probability of class assignment as a quantitative phenotype for analysis. This approach may also reduce heterogeneity and increase the power to identify genetically similar subgroups.
