# Bayes’ Theorem

Learn how to establish a posterior probability based on prior knowledge using Bayes’ theorem.

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## What is Bayes’ theorem?

**Bayes’ theorem** describes the probability of an event based on prior knowledge of conditions that might be related to the event. Named after English statistician Thomas Bayes, it provides a way to revise existing predictions or theories when new evidence becomes available. Bayes’ theorem is used in many fields, from medicine to economics and artificial intelligence to psychology.

Bayes’ theorem can be stated as follows:

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