Panel Accretion and External Validity in Adolescent Substance Use Research

This study investigates the relationship of panel accretion to panel attrition and examines its impact on threats to external validity in adolescent substance use research. Panel accretion involves the addition of new participants to a panel design after initial baseline data has been obtained. Accretion occurs in longitudinal studies in which data is routinely gathered on all participants, as is often the case in prevention or epidemiological research on adolescent substance use. Accretion, attrition, and panel findings for three, 2-year panel samples in grades 7 through 9 are examined. The results indicate that accretion and attrition samples have an isomorphic relationship to one another on a range of dependent measures when drawn from the same population. The results are discussed in terms of their implications for adolescent substance use research, the examination of validity threats due to panel bias, and understanding the ecological validity of research findings.