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Base-Rate Neglect
“A description fits perfectly. How many people of that kind existed before you read it?”
01 / SUMMARY
In short
When judging category membership, people may give too much weight to an individual detail and too little to prior prevalence. Specific evidence matters, but it must be combined with the starting frequency.
03 / SCENE
In the wild
A test returns positive for a rare condition. Without its false-positive rate and prevalence, “positive” does not mean “almost certain.”
04 / LIMIT
The limit
A base rate should not override genuinely strong, diagnostic individual evidence; both pieces belong in the judgment.
FINAL NOTE
What remains
Ask how common the outcome was first, then how far the new evidence should move that figure.