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Click on the arrows to page through the tutorial on the statistical concepts of specificity and sensitivity. In my experience, people get confused when they try to learn it first as a formula and not as a concept. The words sensitive and specific are not difficult, so if you start from there, you will find it is much easier to learn. This tutorial is the joint effort of Profjameson and two Pharm. D. candidates, Caleb Bryant and Nicholas Anderson.
Ideally all tests would be very sensitive and very specific. Unfortunately, that is rarely the case. Sensitivity and Specificity are more or less important depending on the purpose of the test. A broad screening test needs to be fairly sensitive to be of any value. On the other hand a test must be specific to be of value for a definitive diagnosis. That is why the more sensitive (a less specific) ELISA test is used to screen for HIV, but a Western Blot (very specific) test is done to confirm it.
Interestingly, the rapid strep screen is only about 75% sensitive (misses 25% of people who truly have strep), but is fairly specific (very few false positives).