Relative Risk

A comparison of risks as a ratio or percentage change.

Visual intuition

before

2 cases

after

4 cases

Relative risk zooms in on the ratio

Relative risk zooms in on the ratio: doubling can still be tiny in absolute size.

Example

Going from 2 in 10,000 to 4 in 10,000 is a 100% relative increase.

How It Works

Relative risk tells you how much larger or smaller one risk is compared with another. It is useful for comparing strength, but it can sound dramatic when the starting risk is tiny.

Concepts

Absolute vs Relative Risk

A relative change ('50% more', 'doubles your risk') hides how big the underlying risk actually is. A large relative increase on a tiny base rate is still tiny. Always ask for the absolute numbers — how many in 100 before and after — not just the percentage change.

Lessons

A huge whole beside tiny slices that make a percentage look larger than it is.

Ask for the Absolute Number

A percentage change is meaningless without the base it changes.

Related Facts

Risk & Decisions

5 min · medium

Two tiny dots almost the same size, shown as two very different-height bars through a magnifying glass.

Relative risk vs absolute risk: the difference that changes decisions

Relative risk tells you how many times bigger a risk became. Absolute risk tells you how many extra people are affected.