Base-Rate Neglect

“A description fits perfectly. How many people of that kind existed before you read it?”

01 / PLAINLY

What it means, plainly

A vivid individual case can absorb so much attention that we forget how common the outcome was before seeing that detail.

02 / CONTEXT

A little more

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 / WHY IT MATTERS

Why it matters

Without the starting frequency, a striking signal can lead people to misread tests, forecasts, or classifications.

04 / EXAMPLE

A familiar example

A test returns positive for a rare condition. Without its false-positive rate and prevalence, “positive” does not mean “almost certain.”

05 / LIMIT

What it does not mean

A base rate should not override genuinely strong, diagnostic individual evidence; both pieces belong in the judgment.

06 / NOTICE

Notice it in your day

Before classifying a case, write down two things: the outcome's starting frequency and what the new evidence should change. Keep the steps separate.

FINAL NOTE

The idea worth keeping

Ask how common the outcome was first, then how far the new evidence should move that figure.

QUESTIONS / 02

Questions people still have

What is a base rate in everyday language?

It is the starting frequency of an outcome before specific information about the current case is added.

Should the base rate always beat individual evidence?

No. A sound estimate combines prior frequency with the actual strength of the new evidence.