BOX 2-1

Head Start

Head Start is a federally funded school readiness program serving low-income families with young children. Created in 1965, the program focuses on preparing disadvantaged 3-and 4-year-olds for school by providing them with early education and providing their families with support in health, nutrition, and parenting. The services are supported with federal funds and delivered through locally based centers. Studies of outcomes for children who have received Head Start services show benefits that include improved performance on cognitive and academic achievement tests; increased earnings, employment, and family stability; and decreases in use of welfare and involvement with crime.

SOURCE: National Head Start Association (2009).

the Head Start Program). Ludwig noted that this study was designed to be nationally representative, with children who applied to Head Start but were not accepted serving as controls. However, as with most randomized trials, real-world complications have affected the progress of the study. Some of the participants dropped out of the study, and others may not have complied with all of its conditions—not responding to survey questions, for example. Ludwig pointed out that although the response rates in the Head Start study have been good, particularly considering that the program population is very disadvantaged, it is important to ask whether the level of attrition is sufficient to raise cautions about the causal inferences the study was designed to support.

There are several ways to approach that question. One would be to compare the baseline characteristics for the treatment and control groups. However, reassurance that they were basically similar would not provide a complete answer. Each of the baseline characteristics would have its own confidence interval (a measure of the degree of confidence one can have in the value identified, based on sample size and other factors), which suggests some uncertainty about their relative importance in explaining differences across groups and outcomes. In other words, some will be more relevant than others. To address this concern, Ludwig explained, one might use regression adjustment to examine how the estimates change when one does not control for observable baseline characteristics.

A further complication, however, is that some of the attrition in participation could be the result of factors that change over time, after the baseline characteristics are identified. A strategy for addressing that con-

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