Base Rate Neglect: Ignoring the Odds

The Shy Librarian

Tom is shy, loves books, and is very tidy. Is Tom more likely to be a Librarian or a Farmer? Most people say Librarian because he fits the stereotype. But there are 20 times more farmers than librarians. So statistically, it is much more likely that Tom is a shy farmer.

The Medical Test

A disease affects 1 in 1000 people. A test is 99% accurate. You test positive. What is the chance you have the disease? Most say 99%. The real answer is less than 10%. Why? Because out of 1000 people, 1 has it (true positive), but 10 healthy people test positive (false positive). So 1 out of 11 is ~9%.