Many quantities of interest in sociology, psychology, public health, and medicine are unobservable but well-conceived characteristics such as attitudes, temperament, psychological diagnoses, and health behaviors. Such constructs can be measured indirectly by using multiple items as indicators of the unobservable characteristics. The latent class (LC) model can classify individuals into population subgroups based on these unobservable characteristics, by using their responses to questionnaire items that are related to those characteristics. Suppose, for example, we are interested in the construct of nicotine dependence and want to classify respondents into groups corresponding to different types of nicotine dependence we believe are indicated by a number of behaviors relating to cigarette use. We can collect data on several items pertaining to cigarette use and then apply an LC ...