There are unique challenges to estimating causal effects of preschool for students with special needs that have not received attention in the literature. We revisit the Head Start Impact Study (HSIS) to illustrate that when and how special needs is defined has implications for the internal validity of and interpretation of special needs subgroup…

This article synthesizes findings from a reanalysis of data from the Head Start Impact Study with a focus on impact variation. This study addressed whether the size of Head Start's impacts on children's access to center-based and high-quality care and their school readiness skills varied by child characteristics, geographic location, and the…