to use a design that withheld a child’s access to this program. Instead, in selected Head Start sites around the country, 3-year-old children and their families were randomized to one of two conditions: enrolling in a Head Start center at age 3 (early Head Start) or enrolling in the same center at age 4 (later Head Start). Those entering at age 3 were also accepted for enrollment at age 4. About 75 percent of the families enrolled their children in Head Start at the assigned age. Among the remaining 25 percent, some 3-year-olds randomized to early Head Start enrolled at age 4, some randomized to later Head Start enrolled at age 3, and some did not enroll in Head Start at all.

This encouragement trial attempts to modify the time of enrollment in Head Start. If all enrollments matched the assigned condition, standard or intent-to-treat analyses would provide legitimate causal inferences about the effects of the timing of enrollment. Because one-quarter of the parents made enrollment decisions contrary to the assigned condition, the intent-to-treat analysis, which makes no allowance for deviations from the assigned condition, provides a biased estimate of the causal effect of the intervention.

Use of Preventive Interventions to Test and Elaborate Theories of Change and Development

Although using preventive interventions to test and elaborate theories of change and development is the least practical reason for conducting trials, it may be the most important for generating new knowledge. The empirical findings from prevention science experiments can also be used to refine and modify the etiological theories that were used to guide the development of the intervention. Indeed, this bootstrap process—using an incomplete theory to guide the development of an intervention (Sandler, Gersten, et al., 1988) and then using the empirical results to advance the theory and fill in critical gaps—is a hallmark of the current prevention science model. It is also an atypical model in experimental sciences. A traditional epidemiological approach to treatment of an existing disorder, such as schizophrenia, generally uses a randomized trial to test a specific treatment at a certain dosage and length, with the analyses showing whether the treatment had a positive effect. Before conducting a treatment trial using this traditional approach, the hypothesized etiological model is often highly developed, and only when the pharmacokinetics and other factors are well understood is the treatment tested in a rigorous randomized trial.

With modern preventive trials, the experimental trial is, in contrast, often used to inform etiological theory at the same time. An etiological model of drug use, for example, is based on malleable risk and protective factors that can then be targeted by an intervention (Kraemer, Kazdin, et

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