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Chapter 03 · Section II · 16 min read
The Bayes rule
A single equation that explains why a rare-disease test with 95% accuracy mostly produces false positives — and what to do about it.
Bayes’ rule is the single most useful piece of mathematics in this course. It tells you how to update a belief when new evidence arrives.
The intuition: if a disease affects 1 in 10,000 people, and a test for it is 95% accurate, and you test positive — what’s the chance you actually have the disease? Most people guess 95%. The correct answer is closer to 2%. Bayes’ rule is the bookkeeping that gets you there.
This section is a stub. The full version will cover prior, likelihood, posterior — with a worked example using a hypothetical screening programme in Kathmandu.