Survivorship Bias

Overview

Survivorship bias is the error of drawing conclusions from the cases that made it through some selection process while ignoring the cases that did not, because the failures are not visible. The classic example is Abraham Wald's advice in 1943 to armour the parts of returning bombers that showed no bullet holes: the planes hit there had not come back.

Notes

• Every story of a guardian angel, an answered prayer, a miracle cure or a premonition that came true is told by a survivor. The people whose premonitions were wrong, whose prayers went unanswered and whose near-miss was a hit tell no story, so the sample consists entirely of confirmations.
• It also produces the folk wisdom of success: the dropouts who became billionaires are visible, the dropouts who did not are not.
• The cure is to ask what the denominator is. How many people prayed? How many had the premonition? What happened to the rest?

Also see

• Confirmation Bias
• Texas Sharpshooter
• Angels
• Prayer
• Miracles

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