How to Estimate Priors
Estimating priors means finding the right background frequency before vivid new evidence dominates judgment.
Find the Reference Class
Ask what kind of case this is. A medical symptom belongs to a patient population. A news claim belongs to a source track record. A personal conflict belongs to a history of similar interactions. The reference class gives the first anchor.
Use Multiple Anchors
No reference class is perfect. Try several. What is the broad base rate? What is the narrower base rate? What changes if you choose a different but still relevant comparison class?
Beware Convenient Priors
A prior chosen because it protects a desired conclusion is not doing honest work. Good priors may be rough, but they should be chosen because they are relevant, not because they are useful ammunition.
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).