Session 3: Priors, Base Rates, and Likelihoods

Concept Focus

A prior is a starting confidence before new evidence is considered. A base rate is the background frequency of something in a relevant group. A likelihood asks how expected the evidence would be if a claim were true.

The session helps participants avoid being captured by vivid stories. A dramatic example may matter, but it should be judged against the background rate and the diagnostic strength of the detail.

Historical Example

In wartime, governments sometimes overestimated threats because vivid incidents were easier to remember than quieter background data. Good intelligence work requires asking how common an event is and how strongly a new signal changes the estimate.

This illustrates why base rates are not cold or irrelevant. They protect judgment from being ruled by the most dramatic case.

Contemporary Issue

Medical screening tests are a useful modern example. A positive test result can be serious without being certain. The meaning depends on the condition’s base rate and the test’s false-positive and false-negative rates.

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).