National Council on Teacher Quality:

Kenya appears to be a fave location among educational researchers of late. A relatively stable country where teacher salaries are low (primary teachers make the equivalent of about $3,500 annually) must be the draw. To study the effects of ability tracking in schools, three U.S. researchers provided the funding to 121 Kenyan schools so that they could double the number of their first grade teachers, enabling class sizes of 45 students instead of 90.
Half of the students were assigned to the new first grade classes based on their ability, a practice pejoratively referred to in the U.S. as ‘tracking’, and the other half were randomly assigned, regardless of their ability. Researchers found that students in the schools with tracking scored higher–though just a little–on a post-test than their peers in nontracked schools. More important was the fact that the improved performance was consistent across the board at all levels, for low-, medium- and high-scoring students.
Given the inordinate differences between class sizes, the results are likely not generalizable to the U.S., but still of interest.

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