Another math problem that people often get wrong is this. A certain disease strikes 1 out of 1000 people, totally at random. There's a test for the disease that's 95% accurate. If you have the disease, there's a 95% chance the test will say positive, and a 5% chance it says negative, and if you don't have the disease there's a 95% chance the test says negative, and a 5% chance it says positive. You take the test, and it comes back positive. What are the chances you have the disease?
Answer: About 2%.
