Mind & EverydayMental model
A 99 percent accurate test, and you're still probably healthy.
A disease affects 1 in 10,000. The test detects it 99 percent of the time and gives 1 percent false alarms in healthy people. You test positive. Of 10,000 people, one is sick, but about 100 healthy people get a false alarm. Your real chance of illness is around 1 percent. Ignoring the base rate wildly overrates any single result.
Useful: For any test or suspicion, first ask: how common is this thing in the first place?
Source: en.wikipedia.org