Comparison of longitudinal phenotypes based on number and timing of assessments: a systematic comparison of trajectory approaches II.

The popularity of trajectory-based research to characterize developmental courses of alcohol (and other drug) involvement is growing rapidly. Given the increasing use of these methods, there is a need to identify methodological factors that affect course shape and prevalence. Using growth mixture models, the authors characterized the developmental course of 2 indices of alcohol involvement, alcohol use disorder and frequency of heavy drinking, with a prospective sample of 489 young adults (Year 1 age = 18.52; 55% female; 51% with family history of alcoholism) assessed 6 times over 11 years. Then, the authors explored the extent to which trajectory models that eliminated an assessment (at the beginning, middle, or end of the study interval) were similar to the full 6-wave model. Although classifications showed relatively high concordance, trajectory shape and predicted prevalences varied. Misclassification was associated with methodological factors such as probability of class membership and missing data. Findings suggest that researchers thoughtfully consider the nature of the phenomena being studied and the developmental period of interest when designing prospective studies.