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reuse guidelines: [sagepub.com/journals-permissions](sagepub.com/journals-permissions)[ ](sagepub.com/journals-permissions)© 2018 AERA. [http://eepa.aera.net](http://eepa.aera.net)  \nWhat Is in a Definition? The How and When of Special Education Subgroup Analysis in Preschool Evaluations  \nAnna Shapiro  \nChristina Weiland  \nUniversity of Michigan School of Education  \nThere 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 impact estimates. We find that the treatment group in the HSIS was three percentage points more likely to be classified as special education (SD = 0.11, p \u003C.001) at baseline, likely biasing the impact estimates for this subgroup. We also find that the estimated intent-to-treat effects of Head Start on cognitive and socioemotional measures are sensitive to subgroup definition.  \nKeywords: preschool evaluation, special education, measurement  \nSix percent of the preschool-aged children enrolled in early childhood education programs in the 2013–2014 school year received special education services for a diagnosed disability, approximately 90% of whom were served in public programs (Brault, 2011; Chaudry & Datta, 2017; National Center for Education Statistics [NCES], 2016b) . Some of these children enter school with a diagnosis while others, particularly those with disabilities tied to academic functioning, are referred for evaluation by their educators (Hebbler & Spiker, 2016) . To date, more than 80 rigorous studies have examined the effect of preschool on typically developing children’s kindergarten readiness, but only three have examined the effects of preschool on young children with special needs (Duncan & Magnuson, 2013) . Due to this paucity of evidence and the policy goals of early intervention and diagnosis for children with disabilities, two different groups of early childhood experts recently emphasized the critical need for more studies that estimate the causal effects of preschool on children with special  \nneeds (Phillips, Johnson, & Weiland, 2017; Yoshikawa et al., 2013) .  \nTo inform the field’s increasing focus on this group, we revisit data from one of the three studies to date that has estimated the impacts of public preschool on young children with special needs—the Head Start Impact Study (HSIS) . We do so to draw attention to two previously overlooked issues in this area of research that should be taken into consideration in designing future studies—when special needs status should be collected for valid causal inference and how special needs should be defined. Like many early childhood program evaluations, the HSIS measured special needs after children were assigned to a study condition and used only one of several potential indicators of disability to determine special needs status.  \nThe Head Start program, a federal early childhood health and education intervention for lowincome children, is an ideal context in which to study these questions because of its strong emphasis on serving children with special needs  \nShapiro and Weiland  \nand its well-established early screening and diagnosis procedures (Cooke, 1965; Zigler & Meunchow, 1992) . At least 10% of center seats must be reserved for children with special needs, and Head Start centers are required to engage in recruitment activities to locate children with disabilities (Office of Child Development, 1975) . Furthermore, Head Start evaluates all enrollees within 45 days of enrollment using health and developmental screenings, much like the majority of state-funded public preschool programs (Head Start Bureau, 2015; National Inst","cbCaipUuwT4yxtVZ","https://ap.wps.com/l/cbCaipUuwT4yxtVZ","pdf",636227,"English","# Keywords\n## Research focus on special needs definitions\n## Evidence from the Head Start Impact Study (HSIS)\n## Implications for future preschool evaluation research","[{\"question\":\"Why does the timing of special needs measurement matter in preschool evaluations?\",\"answer\":\"Because collecting special needs status after assignment can create imbalanced subgroups, which threatens internal validity and distorts how subgroup impacts are interpreted.\"},{\"question\":\"What does the study find about special education classification at baseline?\",\"answer\":\"In the HSIS, the treatment group was about three percentage points more likely to be classified as special education at baseline, indicating likely bias for that subgroup’s impact estimates.\"},{\"question\":\"How sensitive are Head Start effects to how the special needs subgroup is defined?\",\"answer\":\"The intent-to-treat effects on cognitive and socioemotional measures change depending on the subgroup definition, and literacy, language, numeracy, and externalizing behavior estimates are sensitive to which measure is used.\"}]","What Is in a Definition? The How and When of Special Education Subgroup Analysis in Preschool Evaluations | PDF",1792095114,48]