Answers & ConfidencePrior probabilityPrevalence

Base rate

How common the positive class is in your traffic, which sets the starting probability a decision model has to move away from.

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Category: Answers & Confidence
Also known as: Prior probability, Prevalence
Related terms: 5
Directory entries: 67
Docs: docs.typesafe.ai
Added: 2026-10-12

Definition

The base rate is the fraction of cases that are actually positive before the model sees anything. When positives are rare, a high precision still means many false alarms, and a model that predicts the majority class every time can look accurate while being useless.

It matters for thresholds: the same probability means different things at different prevalences, so a threshold tuned on balanced data can misbehave on production traffic where the base rate differs. Report the base rate next to any accuracy number.

Tagsconfidenceevaluation

Related terms

Definitions that connect to this one.

CalibrationThe property that outcomes given a probability of 0.2 occur about 20% of the time, measured across many predictions.Answers & Confidence ThresholdThe value your code compares against a score, a Noul probability, or a confidence to turn an answer into an action.Answers & Confidence Cost matrixThe table of what each kind of error costs, which turns a probability into the threshold that minimizes expected cost on a given action.Answers & Confidence Confusion matrixThe table of predicted versus actual labels that shows which classes a model actually confuses, not just how often it is right.Evaluation & Training ConfidenceA 0-1 number on every Choice and Score answer that collapses the probability distribution into one certainty score.Answers & Confidence

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