This National Bureau of Economic Research working paper purports to examine the extent and effects of sorting students into classrooms by test scores. It then claims to explore the effect of sorting on overall student achievement as well as on low achievers, high achievers, gifted, special education and Limited English Proficient students. The paper uses standardized Texas state test scores as the measure of learning growth. Based on a comparison between third- and fourth-grade scores, the paper concludes that sorting students by scores is associated with significant learning gains for both lower and higher achievers. It does not, however, find similar effects for the sub-groups. The paper is limited by several important methodological issues. First, it simply assumes, based on test score distributions, that the schools tracked students between classes—and this assumption is highly questionable. Second, it provides no criteria by which students were classified as high or low achievers. Finally, it measures only relative standing of students on two proficiency tests given in different years. It does not measure growth. Because of these and other weaknesses, this paper should not be used to inform policy regarding tracking or grouping practices.