How to Update with New Evidence
Updating well means revising confidence proportionally without becoming volatile or stubborn.
State the Prior
Before considering the new information, state your prior. What did you believe before this arrived, and why? This prevents the new evidence from taking over the whole field simply because it is fresh.
Ask How Diagnostic the Evidence Is
New evidence should move confidence only to the extent that it discriminates between hypotheses. If the evidence is expected under several possibilities, update modestly. If it is much more expected under one possibility, update more strongly.
Record the Movement
Write the movement explicitly: "I was at 0.55; this moves me to 0.68." This makes updating visible and prevents the mind from rewriting its history after the fact.
The Size of the Update Matters
A common mistake is to treat updating as a dramatic conversion. Most evidence should move confidence modestly. A single anecdote, one expert quote, or one surprising event rarely deserves to overwrite a well-grounded prior by itself.
Good updating asks for proportional motion. If the evidence is weak but real, move a little. If it sharply discriminates among hypotheses, move more. If it is expected under several explanations, slow down.
Adjacent References
These are starting points rather than exhaustive citations. They connect this page to nearby work in probability, calibration, cognitive bias, and social epistemology.
- |Thomas Bayes, "An Essay towards solving a Problem in the Doctrine of Chances" (1763).
- |E. T. Jaynes, Probability Theory: The Logic of Science (2003).