BY THE 2000s, hundreds of potential cancer-causing genes had been identified. Everywhere researchers looked, there were more oncogenes and tumor suppressor genes. Presumably, a single mutation in any one of the normal growth-controlling genes could cause cancer. So why wasn’t everybody getting cancer? There is a common issue in surveillance studies called the denominator problem. Suppose we analyze one hundred great baseball players and discover that every one of them has a liver. We might conclude that having a liver makes you a great baseball player. But this is a logical fallacy, because
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