Definition
Plain language
When a score is already near the top of the scale, so there's almost no room left for it to climb higher.
As stated in the literature
A measurement artifact where values bunch against the maximum of a bounded scale, compressing variance and biasing estimated effect sizes downward; the mirror case at the low end is a floor effect.
Also called: ceiling effects, floor effect, floor effects
Why it matters: It matters because a test crowded near its top score hides real differences between people or models, making improvements look smaller than they truly are.
For example, if a test is so easy that almost everyone scores 99 or 100, you can't tell who actually understood the material better.
Heard on the show
“… on to is that the caveats mostly make the numbers conservative, not inflated — the ceiling and floor effects, where a model already agreeing 91% of the time can't rise more than nine points, all run against …”Episode 229 — One Word Flips a Chatbot From Backbone to Yes-Man