Fifty-five South Dakota middle schools were randomly assigned to program or control conditions Schools were orga- nized into three strata by community size and type (city, town, rural community) Blocks of school clusters consisted of 3 clusters from the same stratum located in the same geographic region of the state. Within each block, 1 school cluster was randomly assigned to cach experimental condition. Across blocks, we restricted the allowable assignments to those that reduced the imbalance among experimental cond tions based on district enrolment, in- dex of school academic performance and socioeconomic status, and the existence of a drug prevention program in the district Clusters Schools Cluster constraint Stratified by commu nity size and type. Strata divided according geographical region and then placed in