Why I Take Class Size Research With a Grain of Salt

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John Hattie, in his book Visible Learning, found through his synthesis of many research studies that class size has only a .21 effect size on learning. According to his findings, class size is slightly more effective than charter schools and slightly less effective than comprehensive teaching reforms. Hattie considers the hinge point for a practice to be strongly associated with student achievement at a .40 effect size. Class size does not make the cut.

I can understand these findings. Poor teachers will use poor instruction regardless of the number of kids in their class. Great teachers can move a group of students forward regardless of the odds stacked against them. So why do so many teachers and parents believe class size is crucial to the success of students? Because class size does matter. I am not talking about the research from thousands of classrooms. I am talking about a classroom on any given year.

Calling back to my teaching days, one of my best years I had was with 28 5th and 6th graders in a multi-age classroom. We were a cohesive group of learners. We could have in-depth conversations about the books we were reading or the math problems we were working on. Conversely, there was a year where 21 students gave me a run for my money. The combination of certain personalities made that year a lot more challenging. That classroom of 21 students felt more like 31 students on some days.

You read this and maybe think, “That’s proof that class size really doesn’t matter.” However, I think my example is the exception that proves the rule. First of all, Hattie’s research takes into account hundreds and thousands of studies. This is where social science research is limited; the human factor. As most teachers can attest, one person can change the entire dynamic of a classroom setting. Second, the more students you have, the more closely you follow a script. Group work becomes both physically and socially more difficult when class size gets larger. Hattie acknowledges this (p. 87). He goes on to point out that class size research could largely be based on the fact that teachers don’t readily change their practices when class sizes fluctuate.

The final reason for my skepticism is the research doesn’t acknowledge the long term effects on the teacher. There is a difference between conferring with 20 students and conferring with 30 students. Grading 30 papers is a lot more time consuming than grading 20 papers. Getting 30 students to walk orderly in the hallways is more of a… okay, you get the idea. These little differences, all of these stressors, can add up to larger effects on a teacher’s health and well being. I would wager that there is research out there that shows teachers who regularly have large class sizes are more likely to seek employment elsewhere.

This post is not to question Hattie’s research, but to point out what could happen when people only look at the numbers. My biggest fear is that administrators and school boards will look at this data and say, “You know what? We don’t need that extra teacher. The research says it doesn’t matter all that much.” It does matter. I’ve been there. I suspect that those who disagree have never had to teach in a classroom with 30 students.