This article discusses school-based drug and alcohol prevention programs, and their effect on subsequent use of these substances by students. The article examines programs such as multiple subgroup analysis, post hoc sample refinement, and use of points in time other than the study baseline to calculate attrition rates. The author briefly describes two other common practices, using as examples three of the most widely advocated prevention programs: the Seattle Social Development Project (SSDP), the Life Skills Training (LST) program, and the ATLAS program. (Author)