Inserting the numbers P.A/ D 0:60; P.d jA/ D 0:35; P.d jB/ D 0:25 yields a
value of P.Ajd/ D 0:68.
In the same manner we can determine the probability of having a certain illness
when the test for this illness showed positive. Luckily enough, this is not as large
as one might first expect. Let A be the event ‘the illness is present’ and B the event
‘the test is positive’. The conditional probabilities P.BjA/ and P.BjAN/ yield the
probabilities that a test has been positive for a sick patient and a healthy patient,
respectively.P.BjA/ is called the sensitivity, P.BNjAN/ D 1P.BjAN/ the specificity
of the test.