return to their families. Upon the trial’s completion, intent-to-treat analyses are typically used to assess overall effects as well as examine the conditions under which the intervention effect varies by child, family, or service provider characteristics. To understand how behavior is modified over the longer term by this intervention, the children are typically followed for a year or more beyond the end of the intervention services. Finally, mediation analyses are used to understand how the effects of an intervention actually take place. Both efficacy and effectiveness trials require appropriate analytical models to produce valid statements about intervention effects (Brown, Wang, et al., 2008).

Substantial investment in both time and money is required to conduct a randomized preventive trial. This process begins with framing the theoretical basis for a preventive intervention; then moves on to partnering with communities around an appropriate design, selection and recruitment of the sample, random assignment to intervention conditions, collection of data while adhering to the protocols specified by the design; and finally analysis of data and reporting of the results. The payoff for this work is described in three sections below.

Evaluating the Effects of Preventive Interventions

Some randomized preventive trials examine questions of program efficacy, or impact under ideal conditions, and can also help determine whether the intervention affects hypothesized mediators and proximal targets in expected ways. These efficacy trials are conducted in settings in which the intervention fidelity is maintained at a high level, usually by having trained researchers deliver the intervention rather than by individuals from the community. The intervention itself can be delivered in research laboratory settings outside the community (Wolchik, Sandler, et al., 2002) or in schools or other settings that serve as the units that are randomized to the intervention or control conditions (Conduct Problems Prevention Research Group, 1992, 1999a, 1999b; Reid, Eddy, et al., 1999; Prado, Schwartz, et al., 2009). Efficacy trials require randomization of youth to either the new intervention or to standard settings so that a comparison of outcomes can be made. Some communities have a concern that youth assigned to the control or standard setting do not receive the intervention and thereby do not receive its potential benefit. These concerns can at times be mitigated, as discussed below.

Other randomized trials address questions of effectiveness, or impact under settings that are likely to occur in a real-world implementation of a preventive intervention (Flay, 1986). An effectiveness trial tests a defined intervention that is delivered by intervention agents in the institutions and communities in a manner that would ultimately be used for large-scale

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