Reasoning pattern · probability statistics

Base-rate neglect

Base-rate neglect occurs when prior prevalence is omitted from probability judgments, making rare events or weak diagnostic signals appear more informative than they are.

Also known as: base-rate fallacy

A result can sound dramatic and still change probability only modestly when the underlying event is rare. The corrective move is to include denominators, prevalence and false-positive rates rather than reasoning from vivid cases alone.

In practice

Claims using this reasoning pattern

Contradicted Reviewed Sep 30, 2026

Do vaccines cause autism?

The vaccine-autism claim has been tested in large cohort studies, case-control studies, meta-analyses and evidence reviews. A 2019 Danish cohort of 657,461 children found no increased autism risk after MMR vaccination, including among children with elevated baseline risk, and a 2014 meta-analysis covering more than 1.2 million children found no association with vaccination, MMR, thimerosal or mercury exposure from vaccines. The 1998 paper that catalyzed the MMR claim was a 12-child case series, did not establish causation and was later retracted.

health medicine