How do we know something causes cancer? Aren’t there essentially infinite variables it would be impossible to control for?
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One way is choosing a random sample of test subjects.
Suppose you had 1,000 lab rats, and you divided the group in half, randomly assigning each rat to the control group or the test group. Keep everything the same for both groups, except for the one thing you are testing (like adding tobacco to their diet). If one of the groups has a much higher cancer rate then the other, it’s likely to be the thing you tested. It probably won’t be a genetic predisposition in one group of rats, because about half of those rats should be in each group.
It is actually more complicated, of course, but that’s one good practice in choosing test groups.
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