test for the effect of an intervention even when a community is undergoing major, uncontrolled societal changes, such as a recession. Other designs, for example those that compare a cohort exposed to intervention with the cohort in a previous year, may be more likely to reach erroneous conclusions because of differences between the two groups (e.g., different economic circumstances) that may be undetected or difficult to account for in the analysis.

In prevention science, evaluation trials are usually conducted only after substantial preliminary data demonstrate that the intervention shows promise. Initially a theoretical model of the development of a disorder, or etiology, is used to specify risk and protective factors that can be selectively targeted in preventive interventions. For example, social learning theory posits that for many children, conduct disorder arises from the learned behavior of children exposed to repeated coercive interactions in the family. This etiological theory is then used to identify potential mediators (risk or protective factors), such as inconsistent and punitive parental responses to the child and association with deviant peers, in a causal model for outcomes of substance abuse disorders or delinquency.

A theory of change is then used to identify an existing intervention or to develop a new preventive intervention aimed at these target risk or protective factors. In a program aimed at preventing substance abuse and delinquency among children who are returning to parental care from a foster placement, a parent training intervention might be designed to reduce punitive statements, to enhance communication with the child, and to improve linkages with the child’s own parents and teacher in preparation for the critical transition period of return to the family of origin. The timing of the intervention may be a consideration as well as the content. Key transition periods may occur when a stage of life begins, such as entry into elementary or middle school or during times of stress, such as a parental divorce or separation.

Measures are developed to assess these risk (e.g., punitive and inconsistent parenting) and protective factors (e.g., communication and monitoring of the child over time) to assess the effect of the intervention on parental behavior, and to determine whether changes in these hypothesized mediators actually lead to reductions in deviant behavior among young people.

In a pilot study with a few dozen families, data can be collected to check whether the trainers are delivering the program as designed to the original custodial parents, whether the parents are changing their interactions with their children appropriately, and whether the predicted immediate behavior changes are seen among the children. After successful completion of this initial work, a randomized trial with a larger number of families can then be used to test this preventive intervention on a defined population of foster children (e.g., those in group care) and at a set time preceding their