Archive for January 22nd, 2015

January 22, 2015

Prosecutor’s Fallacy

Lucia de Berk

damned lies

The prosecutor’s fallacy is a fallacy of statistical reasoning, typically used by the prosecution to argue for the guilt of a defendant during a criminal trial (though some variants areĀ utilized by defense lawyers arguing for the innocence of their client). The fallacy involves assuming that the prior probability of a random match is equal to the probability that the defendant is innocent. For instance, if a perpetrator is known to have the same blood type as a defendant and 10% of the population share that blood type, then to argue on that basis alone that the probability of the defendant being guilty is 90% makes the prosecutors’s fallacy.

Consider the case of a lottery winner accused of cheatingĀ based on the improbability of winning. At the trial, the prosecutor calculates the (very small) probability of winning the lottery without cheating and argues that this is the chance of innocence. The logical flaw is that the prosecutor has failed to account for the large number of people who play the lottery.

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