Head Start is the nation's largest school-readiness program for young children considered at risk of poor academic outcomes. Since opening in 1965, the program has served >22 million children and their families. The majority of participants are 3- and 4-year-olds, and almost all live below the federal poverty level. Head Start delivers comprehensive services to meet the needs of the “whole child”: physical and mental health, preschool education, social and emotional growth opportunities, social services for children and families, and parental and community involvement.

A research-and-evaluation component was part of the design of the program, making it a national laboratory for planning and testing a variety of early childhood services. Over the decades, thousands of studies on Head Start and other comprehensive programs have established that early intervention does boost school readiness, although investigations of long-term effects are relatively sparse.

Certainly a major deterrent to productive evaluation is the range...

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