Conditional Probability
The probability of event B occurring given that event A has already occurred, calculated as P(B|A) = P(A ∩ B) / P(A), where P(A) > 0.
Real World
After a positive mammogram result, doctors at the Royal Marsden use conditional probability to reassess the actual likelihood of breast cancer, because the test's false-positive rate changes the calculation significantly.
Exam Focus
Write P(B|A) = P(A ∩ B) ÷ P(A) in full before substituting values — examiners award a method mark for the correct formula even if arithmetic goes wrong.
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