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…

The 1998 reauthorization of Head Start called for a national evaluation of the Head Start program. The goal of Head Start is to improve the school readiness skills of low-income children. Yet characteristics of Head Start programs, such as their quality and the amount of time children spend in them may influence their effectiveness at achieving…