|Overview: Base Rate Fallacy
Focusing on a specific probability while ignoring a more general probability.
What is a Base Rate Fallacy?
John Spacey, updated on February 04, 2018
The base rate fallacy is a tendency to focus on specific information over general probabilities. For example:1 in 1000 students cheat on an examA cheating detection system catches cheaters with a 5% false positive rateAll 1000 students are tested by the systemThe cheating detection system catches SaraWhat is the chance that Sara is innocent?Many people who answer the question focus on the 5% false positive rate and exclude the general statistic that 999 out of 1000 students are innocent. This results in the incorrect answer that there is a 5% chance that Sara is innocent. This incorrect answer neglects the fact that by testing 1000 students, the machine will catch 50 students on average and 49 of these students will be innocent. Therefore, there is approximately a 98% chance that Sara is innocent.
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