Clinician’s Illusion

A doctor at a severe-disease clinic believes the disease is always severe — because she only ever sees the serious cases. The mild ones never walk through her door. Her experience is real, vast, and unrepresentative.

This is the illusion: our picture of anything is drawn from the slice of it that passes in front of us. Teachers of difficult classes think children are unruly. Divorce lawyers think marriage is doomed. Police officers think the city is dangerous. Each is honest; each is looking at a biased sample.

The first principle: experience is not evidence until you know how it was collected. A thousand anecdotes from a skewed doorway are still a skewed picture — just a confident one.

Before you trust your own impression of any group — customers, teenagers, foreigners — ask the doorway question: which members of this group never arrive in front of me? Your sample has a shape. Find it, or it will find you.