What are your thoughts about research that suggests that class size does not matter?

This is NOT what Professor Hattie says. He summarizes PAST experiences (as research focuses on what has happened in the past) and the preponderance of evidence shows that reducing class size DOES INCREASE achievement but the effect is small. Relative to other influences it is a small but positive effect. The key question is to ask “why is the effect of class size so small, particularly given the many claims that we have all made about why it should be much larger?” If your arguments about the influences that really matter were implemented surely it would be easier in smaller classes. Smaller class sizes have been tried many times in many places but the effects remain small.