Bayes' Theorem
A formula for conditional probability: P(A|B) = [P(B|A) × P(A)] / P(B), allowing calculation of P(A|B) when P(B|A), P(A), and P(B) are known.
Real World
During the COVID-19 pandemic, the UK's SAGE committee used Bayes' theorem to calculate the probability that a person actually had the virus given a positive lateral flow test, accounting for the test's sensitivity and the prevalence rate in the population.
Exam Focus
Draw a tree diagram first — label every branch with its probability, then multiply along paths and add for P(B) using the total probability law.
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