Unlike established K-12 public schooling, preschool programs have had to justify their claim for public funding by demonstrating their effectiveness. In the 1960s, longitudinal studies of the Early Training Program, 1 the HighScope Perry Preschool Program, 2 and others showed improvements in intellectual performance that promised to be permanent, but then disappeared a few years later, leading to the perception that the effects of preschool programs were transient. In the 1980s, the Consortium for Longitudinal Studies demonstrated that the known benefits of high-quality preschool programs do not generalize to all preschool programs. To determine reasons for the different outcomes between preschool programs, studies have evaluated various program features, largely focusing on curriculum.12,13 Other preschool program features examined include program quality, 1 program-year at age 4 vs 2 program-years at ages 3 and 4, variability in teacher qualifications, program delivery in classrooms vs homes, and full-day vs part-day programs.Results generally have supported the idea that greater program quality and quantity, such as full-day vs part-day, contribute more to children's development. The study by Reynolds and colleagues 14 in this issue of JAMA investigated full-day vs part-day preschool programs, leveraging their current federally funded opportunity to expand the Chicago Child-Parent Centers. In 11 schools that offered both programs, 409 mostly low-income, minority children attended preschool for a full school day and 573 for part of a day. This study provides data that bear on the current US policy debate about the added value of full-day preschool programs compared with part-day programs.The crucial methodological question of such studies is whether they provide fair comparisons so that group differences in child outcomes are based on group differences in program features rather than children's characteristics. Random assignment of children will achieve such comparability, but this study, like most such studies, 15 did not use this approach because of the high likelihood of nonadherence by parents and school resistance.Instead, principals in consultation with the project team assigned children to full-day preschool using 3 selection criteria. One was "parental preference due to employment or education, transportation barriers, or the lack of available care for the other part of the day." Parental program preference is typical in child care situations: parents seek full-day programs if they must and part-day programs if their circumstances permit. However, this criterion did not lead to assigning all the employed mothers' children to full-day preschool and none to part-day preschool, as might be expected. The groups did not differ significantly in maternal employment-54% of the fullday and 46% of the part-day preschool mothers were employed-making the groups more comparable on this characteristic than would be expected. The small group difference may have been due partly to maternal employment including part-tim...