"Researchers doing observational studies have much the same problem. They try to think of all the differences between two large populations of subjects so that they can statistically negate them so that only the observation in question – the vitamin C level in the example above – is different between the groups. Problem is they can never possibly think of all the differences between the groups. As a consequence, they never have a perfect study with exactly the same number, sex, age, lifestyle, etc. on both sides with the only difference being the study parameter. And so they don’t really ever prove anything. In fact, we would all probably be a lot better off if all the researchers doing observational studies had followed my lead and fallen asleep mid study."