According to the Breast Cancer Surveillance Consortium (BCSC), the sensitivity of mammograms for forty-year-old women is 73 percent. The denominator, P(T), is a bit trickier. A positive test, T, can come both from patients who have the disease and from patients who don’t. Thus, P(T) should be a weighted average of P(T | D) (the probability of a positive test among those who have the disease) and P(T | ~D) (the probability of a positive test among those who don’t). The second is known as the false positive rate. According to the BCSC, the false positive rate for forty-year-old women is about 12
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