Head Start is a federal early childhood intervention designed to reduce disparities in preschool outcomes. The first randomized experimental study of Head Start, the National Head Start Impact Study (NHSIS), found impacts on academic outcomes of .15 to .3 standard deviations measured at the end...

Head Start is a federal early childhood intervention designed to reduce disparities in preschool outcomes. The first randomized experimental study of Head Start, the National Head Start Impact Study (NHSIS), found impacts on academic outcomes of .15 to .3 standard deviations measured at the end...

Many papers use fixed effects (FE) to identify causal impacts of an intervention. In this paper we show that when the treatment status only varies within some groups, this design can induce non-random selection of groups into the identifying sample, which we term selection into identification...