Do test score gaps between advantaged and disadvantaged children originate inside or outside schools? One approach to this classic question is to ask (1) How large are gaps when children enter school? (2) How much do gaps grow later on? (3) Do gaps grow faster during school or during summer? Confusingly, past research has given discrepant answers to these basic questions.We show that many results about gap growth have been distorted by measurement artifacts. One artifact relates to scaling: Gaps appear to grow faster if measurement scales spread with age. Another artifact relates to changes in test form: Summer gap growth is hard to estimate if children take different tests in spring than in fall.Net of artifacts, the most replicable finding is that gaps form mainly in early childhood, before schooling begins. After school begins, most gaps grow little, and some gaps shrink. Evidence is inconsistent regarding whether gaps grow faster during school or during summer. We substantiate these conclusions using new data from the Growth Research Database and two data sets used in previous studies of gap growth: the Beginning School Study and the Early Childhood Longitudinal Study, Kindergarten Cohort of 1998-1999

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Texas provides a unique opportunity to examine teachers without standard university preparation, for it prepares more teachers through alternative pathways than any other state. We find two advantages for mathematics and science teachers prepared in the standard way. First, since 2008 they have been staying in the classroom longer than those who pursued alternative routes. Second, we analyze student performance on Algebra 1 and Biology exams over the period 2012-2018. Algebra I students with experienced teachers from standard programs gain .03 to .05 in standard deviation units compared to students whose teachers were alternatively prepared. For Biology students there are fewer statistically significant differences, although when differences exist they almost all favor standard programs. These effects are difficult to measure in part because teachers are not assigned to teach courses with high-stakes exams at random. Nevertheless, we find strong evidence in Algebra I that students learn more when their teachers have standard preparation. In Biology there is also evidence but less compelling. Thus, we recommend that all states bolster traditional university-based teacher certification, that Texas not take drastic action to curtail alternative certification, and that other states not allow it to grow too quickly.

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