I have been told that .4 effect size represents one year’s growth in one year’s time. Is it really a proportional scale?

Full Question: I am often asked in my school district how to interpret effect size. I have been told that .4 effect size represents one year’s growth in one year’s time. And therefore .8 effect size represents two year’s growth in one year’s time. And therefore 1.2 effect size represents three year’s growth in one year’s time. My question: Is it really a proportional scale?

Answer: “These are very generalised claims but a lot of care is needed – especially the narrowness for breadth of the outcome. It is best to develop expertise within your own school when using effect size, and using internal comparison to help ask questions about what seems to have impact and what did not. While these are over-generalised claims that work ‘in general,’ in the specific they can be very misleading. They are not proportional – indeed they are standard deviation units and range from -3 to +3 usually (but have no bounds) and as Visible Learning shows nearly all effect-sizes in education research tends to be positive (0 to 1.2); i.e. everything (almost) seems to work, so any focus on the magnitude of the effect (which is where effect size make most sense) is good” (John Hattie, personal communication, March 24, 2018).