The Monte Carlo fallacy

Alexander M Owen
Med J Aust 2011; 195 (7): . || doi: 10.5694/mja11.10937
Published online: 3 October 2011

Gambling and diagnostics are related, but strangely reversed, in the way that prior events can affect our clinical judgements

The year was 1913; the location, the roulette tables of a Monte Carlo casino. For the previous 10 spins of the wheel, the ball had landed on black. A red was overdue, so the punters began to bet more aggressively against the trend. But the 11th spin produced yet another black number. As did the 12th, and the 13th ... and the longer the run of blacks continued, the more convinced the gamblers became that the subsequent spin would yield a red. Their wagers accelerated. Their losses snowballed. For it was only after 26 consecutive black numbers (by which time few could afford to continue betting) that the streak finally came to an end. It was perhaps the most profitable night in the casino’s history: records were set, fortunes were lost, and the “Monte Carlo fallacy” was born.

  • Alexander M Owen

  • St Vincent’s Hospital, Sydney, NSW.



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