To plan group-randomized trials where treatment conditions are assigned to schools, researchers need design parameters that provide information about between-school differences in outcomes as well as the amount of variance that can be explained by covariates at the student (L1) and school (L2) levels. Most previous research has offered these…

Background: The Scandinavian countries have a long history of implementing social interventions, but the interventions have not been examined using randomised controlled trials until relatively recently compared with countries like the United States and the United Kingdom. Purpose: The purpose of this paper is to examine the history of randomised…