Many researchers are reluctant to use aggregate data in program evaluation and other policy relevant research. Aggregate data refers to data on individuals that have been averaged by year, by geographic area, by service agency, or in some other way. For example, data on the individual earnings of job training participants may be aggregated by year or student test scores may be aggregated by school. This brief explores when aggregate data, instead of individual-level data, can be used to effectively address social policy research questions. (author abstract)