Primary prevention is a major concern in psychology, but data regarding intervention effectiveness, particularly effect sizes (ES) appear to be lacking. A thorough literature search of Psychological Abstracts, community psychology journals, and textbooks yielded 47 primary prevention articles, 22 of which were data-based, and none of which reported estimates of ES. In terms of data availability, only 13 articles were found for which an ES could be calculated. Reanalysis of 43 significant positive effects from 11 studies revealed a wide variability of ES. Effect sizes ranged from less than 1% to 40%, with a mean of 8%. Data demonstrated a wide variability in the impact of primary preventative effects. The findings suggest that accurate reporting of ES may be valuable in helping to improve the precision of evaluative and decision-making procedures. (Author/NRB)