implementation. This typically requires a stronger community partnership and involvement in all aspects of the study design and conduct. Any community concerns about withholding a new intervention from youth who are randomly assigned to the control or standard condition need to be addressed directly, because of ethical and human subject concerns, as well as from the practical side of maintaining the study design in a field setting. Often, communities come to consider randomization as a fair way to assign a novel intervention program to its community, given insufficient resources to deliver to everyone at once. Communities may want to test one intervention that they have already adopted but not fully implemented; it may be acceptable to compare an enhanced version of this intervention to that already being used (Dolan, Kellam, et al., 1993). Also, for some studies, it may be possible to provide the new intervention later to those who were initially assigned to the control setting (Wyman, Brown, et al., 2008); such wait-list designs, however, allow for only short-term, not long-term evaluations of impact.
An equally important goal of randomized preventive trials is to search for ways to improve in an intervention. A specific intervention that targets a single risk factor, such as early aggressive behavior, can be used in a randomized trial to test a causative link between this risk factor and later behavior or emotional disorders (Kellam, Brown, et al., 2008). Specifically, if one found that the intervention did change the target risk factor, and this led to reduced disorders, it would provide support for the underlying etiological theory. For example, elaborated statistical analyses of intervention impact can show who benefits from or is harmed by an intervention, how long the effects last, and under what environmental circumstances these effects occur. Interventions may deliver different levels of benefit or harm to different kinds of participants or in different environments (Brown, Wang, et al., 2008), and information about these differences can extend the causal theory as well as guide decisions on whether to adopt or expand a prevention program or to attempt to improve outcomes through program modification.
For example, one first-grade intervention was found in a randomized trial to produce improvement in mathematics achievement, but all of this gain occurred among children who began school with better than average mathematics achievement; those who were below average gained nothing compared with children in the control group (Ialongo, Werthamer, et al., 1999). However, a behavioral component of this intervention was found to have a beneficial impact on precursors to adolescent drug use (Ialongo, Werthamer, et al., 1999). In follow-up research studies, the mathematics