Economist Josh Angrist and Victor Lavy shed light on the issue of class size, publishing their results in a 1999 paper. In Israel, classes are capped at 40 students. If one school has 38 fourth-graders, those children will all have one teacher, while in an otherwise similar school that has 42 fourth-graders, the children will be split into two classes with an average size of 21 students. Examining 1991 test scores of students from more than 4,000 fourth- and fifth-grade classes in Israel, Angrist and Lavy found that smaller class sizes generated “significant and substantial” gains in math and reading achievement among the fifth-graders and smaller gains in reading among the fourth-graders.

http://www.technologyreview.com/article/508381/the-natural-experimenter/