In Star Wars, the Jedi suspect that a senator is secretly a Sith Lord. However, there are two possible Sith Lords hiding among the senators.
Only 2% of all senators are Sith Lords.
The Jedi test for Sith corruption:
A senator tests positive for Sith corruption. What is the probability that this senator is actually a Sith Lord, given that there are two Sith Lords among them?