Bayes’ Theorem

Questions

What is Bayes’ Theorem?

Bayes' Theorem describes how to update beliefs when new evidence arrives. You begin with a prior, then shift it according to how strongly the evidence points one way, so strong evidence moves you a lot and weak evidence only a little.

How do you use Bayes’ Theorem?

To use it, write down your starting estimate, ask how common something is before reacting to one result, and change your mind gradually as patterns emerge.

When does Bayes’ Theorem matter?

Bayes’ Theorem is one of the 68 mental models for evidence, the situations where you ask: Is this really true? Within that, it belongs to “Reading the numbers”, alongside Base Rate Fallacy, Central Limit Theorem and Conditional Probability.

What is an example of Bayes’ Theorem?

Two stories show it at work: Turing’s bans (1941) and Nate Silver (2012). Both are told in full in The Mental Models Daily Almanac, which has two stories for each of its 365 models.

Which mental models are related to Bayes’ Theorem?

Probabilistic Thinking, Super Forecasters and Confirmation Bias. Bayes’ Theorem itself comes from statistics and probability.