However, if the development of alcohol use is continuous and discrete alcohol trajectory groups do not exist, why are they generated by statistical approaches in the first place? Surprisingly, trajectories can be derived from nonnormal data—and even appear optimal—when only one group exists in the population.1,2 Growth mixture models require conditional normality, and even small violations of this assumption will lead to the extraction of too many latent classes. If no real population subgroups exist, these classes are invalid; if they do exist, there is little chance that they will be accurately mapped by the estimated trajectories.13,17 Because the distribution of alcohol use is nonnormal throughout development, trajectory analyses of alcohol use will violate the normality assumption and may lead growth mixture models to extract artifactual latent classes.
