Regression to the Mean
Etymology
Described by Sir Francis Galton in the 1880s, who first called it "regression toward mediocrity" after noticing that tall parents tend to have somewhat shorter children. "Regression" is from Latin "regressus" ("a return"), from "regredi" ("to go back").
noun phrase · /rih-GRESH-un too thuh MEEN/
The tendency for an extreme measurement to be followed by one closer to the average, purely by chance.
What it names
An essential guard against fooling yourself: exceptional results often drift back toward normal on their own, so don’t credit (or blame) whatever you did in between.
Example
The star rookie’s slump the next year was mostly regression to the mean, not a coaching failure.
Related idea
It powers the post hoc fallacy: a treatment given at someone’s worst moment gets false credit for the improvement that regression would have produced anyway.