The available evidence suggests a significant rate of attrition in both types of studies. Attrition in school-based studies is generally due to student absenteeism, student transfers, drop-outs between initial interview and postintervention assessment, failure by the student to come to the assessment, decision by the student or parent to discontinue participation, or failure to complete the questionnaire properly. Subject attrition can threaten the internal validity of a study that depends on the researchers' confidence that any differences between experimental conditions are due to the experimental variable that was being studied rather than to extraexperimental factors. The authors recommend randomizing the units of study in order to maximize the equivalence of attrition across groups. Attrition also threatens a study's external validity to the degree that subjects who are missing from postintervention assessments are systematically different from the remaining subjects. However, the authors note that using schools, rather than individual students, as the unit of analysis can obviate the effects of attrition. Attrition rates can be reduced if researchers try to attain the largest possible proportion of students, return to assess absentees, and track missing students. 2 tables, 20 references, and 1 appendix