Prevalence

The share of a population that has a condition at one moment in time.

Visual intuition

3 in 100

before new evidence

Prevalence: 3 of these 100 have it right now

A snapshot of 100 people: the 3 highlighted have the condition at this moment — a prevalence of 3%. That is also the base rate a test faces, before any new evidence arrives.

Formula

prevalence = people who have it now / people in the population

Example

If a town of 10,000 has 300 people living with diabetes today, the prevalence is 3% — whether they were diagnosed last week or twenty years ago (illustrative numbers).

How It Works

Freeze a at one instant and count who currently has the condition: 3 of these 100 people means a prevalence of 3%. It is a stock, not a flow — long-lasting conditions pile up to high prevalence even when new cases are rare, and quickly-cured conditions stay low-prevalence even when new cases are common ( measures those new cases). Prevalence is also the a test walks into: it sets how many true cases exist for a positive result to point at.

Concepts

Base Rates

The background frequency of something before you consider new evidence. A strong signal can still produce many false alarms when the thing being tested for is rare, because there are many more non-cases than cases. Good reasoning combines the new signal with the base rate.

Lessons

A huge field of common dots with only a few rare dots under a magnifier.

Rare Things Stay Rare

Even strong evidence can mislead when the thing you are testing for is very uncommon.

Related Facts

Risk & Decisions

4 min · medium

A magnifier over a large crowd where only a few figures are coral, the rest teal.

A good test can still scare too many people

When a condition is rare, most positive test results can still be false positives.