# Posterior probability

https://arbital.com/p/posterior_probability

by Eliezer Yudkowsky Jan 27 2016 updated Jul 10 2016

What we believe, after seeing the evidence and doing a Bayesian update.

"Posterior probability" or "posterior odds" refers our state of belief after seeing a piece of new evidence and doing a Bayesian update. Suppose there are two suspects in a murder, Colonel Mustard and Miss Scarlet. Before determining the victim's cause of death, perhaps you thought Mustard and Scarlet were equally likely to have committed the murder (50% and 50%). After determining that the victim was poisoned, you now think that Mustard and Scarlet are respectively 25% and 75% likely to have committed the murder. In this case, your "prior probability" of Miss Scarlet committing the murder was 50%, and your "posterior probability" after seeing the evidence was 75%. The posterior probability of a hypothesis $H$ after seeing the evidence $e$ is often denoted using the conditional probability notation $\mathbb P(H\mid e).$