in any sort of test. For instance, the sensitivity of a Pregnancy test
is expressed as the percentage of pregant women who get a positive
result. The specificity of the test is the percentage of non-pregnant women who get a negative
Perfect sensitivity and perfect specificity are usually mutually exclusive, and so compromises must be made in test design, usually favoring safety. AIDS tests, for example, aim to give positive results to all testees who have AIDS, and end up giving false positives to a non-trivial number of non-sufferers. Further rounds of tests can help get rid of the false signals.