Suppose that based on symptoms a patient has, his doctor is
60% certain that the patient has a
particular disease. If doctor's suspicions would be
overwhelming, say at least 85%, then he would
recommend a surgery. Under these circumstances, the doctor
opts for quite an invasive and
expensive procedure, which unfortunately is not 100% reliable.
In particular, the test can show
positive even if the patient does not have the disease (false
positive), because of his diabetes.
Chances of this is 30%. On the other hand, the test can show
negative if the patient does have the
disease (false negative) in 10% of all cases. Question is, in
the event the test shows positive, how
much higher the prior estimate of 60% should increase to make
the test worth rendering. Can we
accurately predict the results of this before running the test
in order to see if a positive test will
elevate the prior from 60% to 85% or higher?