Base rate fallacy

<p>The base rate fallacy, also called base rate neglect or base rate bias, is a type of <a href="/facts/Fallacy/IRVuLSef">fallacy</a> in which people tend to ignore the <a href="/facts/Base_rate/HnRytNkF">base rate</a> (e.g., general <a href="/facts/Prevalence/6UTuTGTH">prevalence</a>) in favor of the individuating information (i.e., information pertaining only to a specific case). For example, if someone hears that a friend is very shy and quiet, they might think the friend is more likely to be a librarian than a salesperson. However, there are far more salespeople than librarians overall—hence making it more likely that their friend is actually a salesperson, even if a greater proportion of librarians fit the description of being shy and quiet. Base rate neglect is a specific form of the more general <a href="/facts/Extension_neglect/976wHch4">extension neglect</a>.
</p><p>It is also called the prosecutor's fallacy or defense attorney's fallacy when applied to the results of statistical tests (such as DNA tests) in the context of law proceedings. These terms were introduced by William C. Thompson and Edward Schumann in 1987, although it has been argued that their definition of the prosecutor's fallacy extends to many additional invalid imputations of guilt or liability that are not analyzable as errors in base rates or <a href="/facts/Bayes%2527s_theorem/wQqFB2Dt">Bayes's theorem</a>.
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Base rate fallacy open-in-new

Base rate fallacy