Most tests are probabilistic and you have to think about the conditional probability given other indicators. Consider a urine culture for detecting UTIs. If you

Most tests are probabilistic and you have to think about the conditional probability given other indicators. Consider a urine culture for detecting UTIs. If you just get urine samples from a general population, some fraction will show positive (bacteria in the culture) falsely. It would be pointless to perform this test on the general population and to treat people that show positive . But if you perform this test conditioned on the fact that the patient is presenting other symptoms (burning sensation, fever etc.) then the same test very useful and the conditional false probability is low enough that it can be used to decide on treatment. The same idea applies to most tests in medicine. That is why tests are mostly done conditionally.