Bayes' rule: Guide

Bayes' rule or Bayes' theorem is the law of probability governing the strength of evidence - the rule saying how much to revise our probabilities (change our minds) when we learn a new fact or observe new evidence.

You may want to learn about Bayes' rule if you are:

• A professional who uses statistics, such as a scientist or doctor;
• A computer programmer working in machine learning;
• A human being.

As Philip Tetlock found when studying "superforecasters", people who were especially good at predicting future events:

The superforecasters are a numerate bunch: many know about Bayes' theorem and could deploy it if they felt it was worth the trouble. But they rarely crunch the numbers so explicitly. What matters far more to the superforecasters than Bayes' theorem is Bayes' core insight of gradually getting closer to the truth by constantly updating in proportion to the weight of the evidence.

— Philip Tetlock and Dan Gardner, Superforecasting

Learning Bayes' rule

This guide to Bayes' rule uses Arbital's technology to allow for multiple flavors of introduction. They vary by technical level, speed, and topics covered. After you pick your path, remember that you can still switch between pages, in particular by using the "Say what?" and "Go faster" buttons.