# Base Rate **Domain:** Statistical inference **Doc Type:** Developed Concept Node **Maturity:** Developed **Collection:** [[collections/Gamification|Gamification Collection]] ## Definition A **base rate** is the prevalence of a condition in the population before a new signal or test result is considered. Even a seemingly accurate classifier can produce many false positives when the target condition is rare. ## Why it matters Extremism screening, fraud detection, bot classification, and eligibility review must report base rates alongside sensitivity and specificity. Without prevalence, “accuracy” can conceal the number of innocent or irrelevant cases routed into intervention. ## Sources - NIST, [Sensitivity, specificity, and predictive values](https://www.itl.nist.gov/div898/handbook/prc/section2/prc241.htm) ## Routes [[wiki/False Positive|False Positive]] · [[wiki/Cost Matrix|Cost Matrix]] · [[wiki/Prevent|Prevent]]