Effect size
MCAT glossary · Critical Analysis & Reasoning Skills
Definition
Magnitude of an effect independent of sample size (Cohen's d, r); large effect: d ≥ 0.8.
Effect size in plain English
Effect size answers 'okay, it works — but does it matter much?' A study can prove a difference is real without that difference being big enough to care about. Effect size measures the actual strength of a result — like the difference between a pill that lowers your fever by 5 degrees versus one that lowers it by half a degree; both might be 'real,' but one clearly matters more. Crucially, it doesn't get inflated just by testing more people, so it's an honest measure of magnitude; a Cohen's d of 0.8 or higher counts as a large, meaningful effect.
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