# Probability Theory: Bayes’s theorem

### Probability Theory: Bayes’s theorem

Bayes’s theorem used for evaluating the accuracy of a medical test [Credit: Encyclopædia Britannica, Inc.]Consider now the defining relation for the conditional probability P(An|B), where the Ai are mutually exclusive and their union is the entire sample space. Substitution of P(An)P(B|An) in the numerator of equation (4) and substitution of the right-hand side of the law of total probability in the denominator yields a result known as Bayes’s theorem (after the 18th-century English clergyman Thomas Bayes) or the law of inverse probability: As an example, suppose that two balls are drawn without replacement from an urn containing r red and b black balls. Let A be the event “red on the first draw” and B the event “red on the second draw.” From the obvious relations P(A) = r/(r + b) = 1 − P(Ac), P(B|A) = (r − 1)/(r + b − 1), P(B|Ac) = r/(r + b − 1), and Bayes’s theorem, it follows that the probability of a red ball on the first draw given that the second one is known to be red equals (r − 1)/(r + b − 1). A more interesting and important use of Bayes’s theorem appears below in the discussion of subjective probabilities.

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