How are pretest and posttest probabilities related?

How are pretest and posttest probabilities related?

• A. Posttest equals pretest
• B. Posttest is independent of test results
• C. Posttest probability is updated by test results
• D. Pretest and posttest are identical concepts

Answer: (C)

Updating beliefs after new information from a test is the idea here. You start with a pretest probability—the estimate of how likely the event or condition is before testing. When a test result comes back, that result provides new evidence, and the probability is adjusted based on how reliable the test is (its sensitivity and specificity). If the result is positive and the test is reasonably accurate, the posttest probability goes up; if the result is negative, the posttest probability goes down. The size of the change depends on the test’s accuracy, often captured conceptually by likelihood ratios. So the correct idea is that the posttest probability is updated by the test results. The other statements don’t fit because the probability would not remain the same after testing, would not be independent of the test result, and the pretest and posttest are not identical—they refer to probability before and after incorporating the test information.

Updating beliefs after new information from a test is the idea here. You start with a pretest probability—the estimate of how likely the event or condition is before testing. When a test result comes back, that result provides new evidence, and the probability is adjusted based on how reliable the test is (its sensitivity and specificity). If the result is positive and the test is reasonably accurate, the posttest probability goes up; if the result is negative, the posttest probability goes down. The size of the change depends on the test’s accuracy, often captured conceptually by likelihood ratios.

So the correct idea is that the posttest probability is updated by the test results. The other statements don’t fit because the probability would not remain the same after testing, would not be independent of the test result, and the pretest and posttest are not identical—they refer to probability before and after incorporating the test information.