Screening's Strange Math

This hypothetical example of a test illustrates the arithmetic involved with medical screening, false positives, and creating sound guidelines.

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Organizations formulating guidelines to ensure that tests are doing more good than harm must consider how often tests produce false-positive results. The U.S. Preventive Services Task Force, for example, has concluded that almost 2,000 women in their forties would have to be screened to prevent one death from breast cancer, and this could produce as many as 200 false positives, sometimes leading to unnecessary biopsies and even, in some cases, unnecessary surgery, chemotherapy or radiation. This hypothetical example of a test, put forth by mathematician John Allen Paulos, illustrates the type of arithmetic involved.

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