See the article published on this topic:

West, S. G. & Aiken, L. S.  (1997).  Toward understanding individual effects in multicomponent prevention programs:  Design and analysis strategies.   In Bryant, K. J., Windle, M., & West, S. G. (Eds).  The Science of Prevention:  Methodological Advances From Alcohol and Substance Abuse Research.   American Psychological Association:  Washington, D. C., pp. 167-209.