Subject attrition in prevention research.

Subject attrition threatens the internal validity of substance abuse prevention studies because differences in the rate of attrition and the substance use behavior of remaining subjects in the different conditions could account for any differences found in substance use rates. Attrition threatens the external validity of prevention studies because, to the extent that study dropouts are different from remaining subjects, the results of the study may not be generalizable to study dropouts. Analysis of these threats to the validity of prevention studies should be routinely conducted. However, studies of alcohol and drug abuse prevention have generally failed to report or analyze subject attrition. Smoking prevention studies have more frequently reported attrition, and they have recently begun to analyze the degree to which attrition may affect the internal and external validity of the study. Evidence thus far suggests that differences in attrition across conditions do occur occasionally. The evidence is substantial that study dropouts are systematically more likely to smoke, to use other substances, and to score highly on other risk-taking measures.