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
“The boring explanation is a ceiling effect.”Episode 015 — The Audit Number Isn't What You Think: Sycophancy and the Case Against Single-Prompt Bias Tests