Practise · The base-rate square
A positive test. How worried should you be?
A puzzle about tests of every kind. Guess first. Then count the squares.
One person in a hundred has a condition. A test catches nine in ten of them. It also flags five in a hundred healthy people.
You test positive. What is the chance you have it?
Of everyone who tests positive, the share who have it:
Your sealed guess:
Most positives come from the large healthy group. A small error on many people beats a high hit rate on few.
- Has it and tests positive
- Healthy but tests positive
- Has it but the test misses it
- Healthy and tests clear
A thinking exercise, not medical advice. Ask a doctor about a real result.