These numbers come from something called the Nurses Health Study II, long-term project out of Harvard on how lifestyle affects disease risk. Basically, they took a big cohort of nurses several decades ago, asked them a ton of questions about their lives, and then continued to follow up with them periodically. They asked the same questions and performed the same tests, and over time researchers have been able to find associations between particular lifestyle factors and particular diseases. It’s important to remember, though, that they’re just associations. And that the subjects are all nurses. Occupation might not be relevant for studies about diet or exercise, but it’s important when we’re looking at exposure.