Research: Sorting Students By Ability Increases Achievement

Sorting students by ability substantially helps their achievement in mathematics and reading, according to a new study from the National Bureau of Economic Research. The authors looked at student performance in schools that sort their classrooms by skills and those that do not and compared the outcomes.

Sorting students has long been controversial because of the interplay of two social forces in effect in classrooms. One is the “tracking effect” which allows teachers to focus more narrowly on a group of students with particular set of needs. The other is the “peer effect” where students could be influenced to perform better by the higher achieving classmates around them.

According to Courtney A. Collins and Li Gan, who authored the report, high-performers do equally well in mixed and sorted classrooms because they simultaneously benefit from the peer and the tracking effects. Yet those who fall behind while still getting a measure of benefit from the tracking effect in the form of additional teacher attention will only fall further behind when relying on the peer effect, making a homogeneous classroom a bad idea for students with those performance characteristics.

Analysts attempt to account for unobservable ways that schools might sort (say, by student behavior) and ultimately find that three-quarters of the schools organize students along at least one dimension: Nineteen percent by prior math scores, 24 percent by prior reading scores, 28 percent by “gifted” status, 57 percent by LEP (limited English proficiency) status, and 13 percent by special-education status (further, around 40 percent sort by at least two dimensions).

Both groups of students – high performers and low – benefited by a classroom sorted by achievement. Although high performers enjoyed the lion’s share of the benefits, all groups benefited from being in non-homogeneous classrooms.

We use a unique student-level data set from Dallas Independent School District that links students to their actual classes and reveals the entire distribution of students within a classroom. We find significant variation in sorting practices across schools and use this variation to identify the effect of sorting on student achievement. Implementing a unique instrumental variables approach, we find that sorting homogeneously by previous performance significantly improves students’ math and reading scores. This effect is present for students across the score distribution, suggesting that the net effect of sorting is beneficial for both high and low performing students. We also explore the effects of sorting along other dimensions, such as gifted and talented status, special education status, and limited English proficiency.