The Base Rate Trap

A factory scanner catches 99 percent of faulty parts and wrongly flags only 1 percent of good ones. It flags your part. How worried should you be?

It depends on a number nobody mentioned: how common faulty parts are. Say 1 part in 1,000 is faulty. Picture 100,000 parts.

• 100 are faulty, and the scanner flags 99 of them.
• 99,900 are fine, and the scanner wrongly flags 1 percent, which is 999.

That makes 1,098 flagged parts, of which only 99 are truly bad. Your flagged part has about a 9 percent chance of being faulty.

Why it trips everyone up

A scanner that is right 99 percent of the time sounds like it should give you a 99 percent answer. But when the thing you are hunting is rare, the small error rate on the huge pile of good parts swamps the real finds.

This is the base rate: how common something is before any evidence arrives. Ignoring it is called base rate neglect.

Use it this week

When you see a confident claim about a rare event, picture a thousand or a hundred thousand actual cases and count them. Counting whole things is far kinder to the brain than juggling percentages.