Reducing Class Size

News blog readers know that from time to time I make a diversion into the territory of evidence-based education. On the way home from work recently, I listened to a discussion about the merits of reduced class size. One of the protagonists argued that reducing class size was very beneficial to learning outcomes. The other said that educational outcomes were hardly affected by class size. So I turned again to Hattie’s monumental work.[1] There was support for both positions from this well-studied intervention; the debate concerns the magnitude of the effect. The total number of students across the studies is about 1 million and the effect of reducing class size from about 25 to about 15 is about 0.15 of a standard deviation. This might sound like a nugatory effect (as argued by one of the debaters). However, a standard deviation of this magnitude represents about half a year of learning achievement. Remember, a standard deviation of only 0.3 represents a whole year and a standard deviation of 1.0 represents going on for three years of achievement, on average. Reducing class size is much less effective than many other interventions, but it still seems highly desirable. There is also an argument that teacher satisfaction and retention might be improved by smaller class sizes. However, when all is said and done, class-size is not nearly as important as teacher ability (which in full is not nearly as important as student ability, but that is a given for any particular class).

— Richard Lilford, CLAHRC WM Director


  1. Hattie J. Visible Learning: A Synthesis of Over 800 Meta-Analyses Relating to Achievement. Oxon, UK: Routledge, 2009.

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