Base-Rate Neglect

We ignore background statistical frequencies (base rates) when given specific, vivid information about an individual case — even when the base rate is the most relevant fact.

• A doctor told that a patient 'seems depressed and withdrawn' diagnoses depression — ignoring that the base rate for clinical depression in healthy adults is low and many things cause withdrawal.
• Given a description of a quiet, bookish man, people say he's more likely a librarian than a farmer — ignoring that farmers vastly outnumber librarians.
• Investors treat a startup with a compelling pitch as highly likely to succeed — ignoring that 90% of startups fail regardless of how good the pitch sounds.