think about a sample of a thousand women. Out of every 1,000 women, 10 have breast cancer (that’s the prevalence, or base rate). Of these 10 women who have breast cancer, 9 will test positive (that’s the test’s sensitivity). Of the 990 women without breast cancer, about 89 will nevertheless test positive (that’s the false-positive rate). A woman tests positive. What is the chance that she actually has breast cancer? It’s not that hard: 98 of the women test positive in all, 9 of them have cancer; 9 divided by 98 is around 9 percent
