Fallacy · Bad evidence
Base rate fallacy
Also called: Base rate neglect
Ignoring how common something is in general when judging a particular case.
What it is
Specific information grabs attention, but the background rate still matters. A test that is right 99% of the time can still give mostly false alarms if the condition it looks for is very rare, because the rare true cases are outnumbered by the errors among everyone else.
An example
A quiet, tidy person who loves books is described. Is she more likely a librarian or a salesperson? Many say librarian, but there are far more salespeople, so even many tidy book-lovers are salespeople.
How to spot it
A vivid description or test result is weighed without asking how common the thing is to begin with.
How to answer it
“Before the details, how common is this in the first place?”
When it is not a fallacy
Specific evidence can rightly outweigh the base rate when it is strong enough. The fair move is to start from how common the thing is and let the evidence shift the estimate, rather than ignore either.
Printable sheet of all fallacies
Cite or share this page
A plain reference you can paste into notes or an essay:
Quest2Reason. “Base rate fallacy.” https://quest2reason.com/learn/ideas/fallacies/base-rate-fallacy. Accessed 13 November 2026.