To address the issues regarding quality thresholds, features, and dosage identified in the literature review, secondary data analyses were conducted using data from eight large-scale ECE research projects. Analyses were conducted in parallel across as many data sets as possible that met our criteria for inclusion in order to identify patterns that were replicated across data sets and across types of center-based ECE (including Head Start, prekindergarten, and community-based child care). All studies included in these analyses focused primarily on children from low-income families. Most studies included only children enrolled in Head Start or public prekindergarten programs, but a few included children in community-based child care. Head Start primarily serves children from low-income families--although the program requires 10% of enrollment to comprise children with special needs, regardless of family income. Most state prekindergarten programs restrict service to or favor the recruitment of children from low-income families. In contrast, community-based child care tends to serve families from a wide variety of socioeconomic backgrounds. (author abstract)