Bayesian Belief Updater

Set a prior probability, feed in evidence events, and watch the posterior update in real time via Bayes' theorem.

Feb 23, 2026, Eric

The Core Idea

Every piece of evidence should move your beliefs — but by how much depends on how likely that evidence is given each hypothesis. Bayes' theorem formalizes this: P(H|E) = P(E|H) × P(H) / P(E). This tool makes that updating process interactive and visible, showing how a prior belief shifts as evidence accumulates.

Prior Belief

P(Bullish) = 60%

1% (strong bear)50% (neutral)99% (strong bull)

Evidence Events

Earnings beat (>5%)P(E|Bull)=70%P(E|Bear)=30%

Strong jobs reportP(E|Bull)=65%P(E|Bear)=35%

Add from library:

Add custom evidence:

P(E|Bull)%

P(E|Bear)%

Belief Update

Prior60.0% bullish

60%

50%