Base rate neglect

Base rate neglect is when you ignore how common or rare something is in general and focus only on the specific clue in front of you. The overall frequency in the population gets underweighted, so you can end up believing something is likely when it is still rare. It shows up in medical, legal, and everyday judgments.

Examples

• A test for a rare disease is 95% accurate. You test positive and assume you likely have the disease, without considering that because the disease is rare, most positive results are still false positives.

• You meet a quiet, bookish person and assume they are a librarian, ignoring that there are far more people in other jobs who are also quiet and bookish.

• A startup has a flashy pitch, so you assume it will succeed, paying little attention to the high base rate of startup failure.

• A witness picks a suspect out of a lineup; the jury treats that as strong evidence, underweighting how often eyewitnesses are wrong even when confident.

• Your child is very tall for their age, so you assume they will be a professional athlete, without considering how few people ever make it to that level.