It is important that evaluations of positive youth development programs undertake their investigation with sufficient sample size. Sample sizes must be large enough that any programmatically significant impact is also statistically significant. When examining the impact of a program, subsamples must often be analyzed, and the size of each of these subsamples must also be large enough to show statistically significant differences where they exist. This is particularly challenging when investigators wish to assign communities, classrooms, schools or districts to experimental conditions. If they want the unit of assignment to line up with the unit of analysis, they must then contend with the statistical implications of the decision. Such choices almost inevitably produce smaller sample sizes than if they assigned individual participants to conditions. All the effective, well evaluated programs used samples of sufficient size, ranging from at least 100 per experimental and control group to, in a number of the school-based interventions, more than 1000 per condition. Only a few positive youth program evaluations analyzed here had total samples of fewer than 200 participants, and only one had a total sample of less than 100.