Definition
Plain language
A number that says not just whether a difference is real, but how big it is.
As stated in the literature
A standardized measure of the magnitude of an observed difference or relationship (e.g., Cohen's d, correlation coefficient), independent of sample size; distinguishes practically meaningful effects from statistically detectable but trivial ones.
Also called: effect sizes
Why it matters: It stops people from getting excited about differences that are technically detectable but too small to make any practical difference.
For example, a new tutoring method might raise test scores by a tiny sliver that shows up in a huge study, and the effect size tells you that sliver is too small to bother with in a real classroom.
Heard on the show
“And the effect sizes on the headline result?”Episode 011 — When RL Actually Teaches Agents Something New, And When It Doesn't