Class size effects on student achievement in Norway: Patterns and explanations

The causal effect of class size on student performance is investigated using a maximum class size rule of 30 students to generate an instrument for actual class size. The data come from lower secondary schools in Norway, and the initial analysis reveals a small negative class size effect. Further investigations show that the effect varies among student subgroups and that the benefits of smaller classes are larger in schools with a high proportion of students who come from intact families. The second section of the paper uses information about the teachers' grading practices and the students' efforts in class to shed additional light on the revealed patterns of class size effects. Class size effects seem to be conditional on student effort.