In the current study, we use data for the first follow-up year of the Head Start Head Start Impact Study to examine variation in Head Start's impacts on children. Specifically, we examine whether there is statistically significant variation in Head Start's impacts on children's cognitive and socio-emotional outcomes across individual children, subgroups of children, and Head Start centers. To do so, we use a new and innovative methodology for estimating sources of variation in program impacts (Bloom, 2012; Bloom, Raudenbush, & Weiss, 2013). (author abstract)