How to use this calculator
Enter the prior and two conditional probabilities, then choose a natural-frequency base.
Formula and method
P(A|B)=P(B|A)P(A)/[P(B|A)P(A)+P(B|not A)P(not A)].
Worked example
Prior .01, sensitivity .9, false-positive rate .05 gives posterior .009/(.009+.0495)=0.153846.
Assumptions and supported scope
The conditioning event must have positive probability. Inputs describe one binary event and one binary observation; counts are expected, not observed. No diagnostic advice.
Method references
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