Why I Take Class Size Research With a Grain of Salt


photo credit: dcJohn via photopin cc

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.

Author: Matt Renwick

Matt Renwick is an 18-year public educator who began as a 5th and 6th-grade teacher in Rudolph, WI. He now serves as an elementary principal for the Mineral Point Unified School District (http://mineralpointschools.org/). Matt also teaches online graduate courses in curriculum design and instructional leadership for the University of Wisconsin-Superior. He tweets @ReadByExample and writes for ASCD (www.ascd.org) and Lead Literacy (www.leadliteracy.com).

3 thoughts on “Why I Take Class Size Research With a Grain of Salt”

  1. I just want to say thank you for all the time you put into your blog posts. I truly enjoy hearing your insights from a principal’s perspective and there always seems to be something that really hits home with what you have to say. I do not work any where near your district but still find everything you post wonderful and often inspiring. I appreciate knowing that there is someone in administration that truly remembers and gets what it is like to be the teacher in the classroom. Thanks for all you do! I hope your staff appreciates who you are as an administrator!


  2. I completely agree with your paper on class size. It is one of Hattie’s effect sizes with which I disagree. At my school without employing an extra teacher we were faced with class sizes of 30 in stage 2 and stage 3 and a 3/4/5/6 multi-age class. The only way this would have worked would be to form a gifted and talented class which would have removed the good role models from the other classes. Yes, the quality of the teaching is crucial but it is a harder slog with a larger class.


    1. Thank you for your comment. The situation you describe sounds frustrating. Hattie’s research is essential to have when making schoolwide instructional decisions, even class size. What I struggle with is the possibility that an administrator or a district might look at his data only and not the additional narrative. We also need to employ common sense when staffing. This is why I prefer the phrase “data-informed” instead of “data-driven”. We, the professionals, need to be in the driver’s seat when making decisions on behalf of kids!


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