Prior Probability
The probability P(A) before observing any evidence related to A, representing initial belief or background information.
Real World
Before any blood test results arrive, a GP estimates there is a 2% chance a patient has Type 2 diabetes based on UK population prevalence data from NHS Digital — this 2% is the prior probability.
Exam Focus
Identify the prior as the probability before new data is observed — examiners award marks for distinguishing prior from posterior clearly.
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