Every diagnostic challenge in medicine can be imagined as a probability game. This is how you play the game: you assign a probability that a patient’s symptoms can be explained by some pathological dysfunction—heart failure, say, or rheumatoid arthritis—and then you summon evidence to increase or decrease the probability. Every scrap of evidence—a patient’s medical history, a doctor’s instincts, findings from a physical examination, past experiences, rumors, hunches, behaviors, gossip—raises or lowers the probability. Once the probability tips over a certain point, you order a confirmatory test—and then you read the test in the context of the prior probability.

Every diagnostic challenge in
medicine can be imagined as a probability game. This is how you play the game: you assign a probability
that a patient’s symptoms can be explained by some pathological dysfunction—heart failure, say, or
rheumatoid arthritis—and then you summon evidence to increase or decrease the probability. Every scrap
of evidence—a patient’s medical history, a doctor’s instincts, findings from a physical examination, past
experiences, rumors, hunches, behaviors, gossip—raises or lowers the probability. Once the probability
tips over a certain point, you order a confirmatory test—and then you read the test in the context of the
prior probability.