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
“It documented two of its own instrumentation bugs with before-and-after numbers, revised its effect sizes downward, and addressed a published result that points the other way.”Episode 233 — Why a Model Can Grade an Answer But Not Write the Answer Key