Nowadays we call this a controlled experiment. The principle is simple. To understand the causal effect of the diet, we would like to compare what happens to Daniel on one diet with what would have happened if he had stayed on the other. But we can’t go back in time and rewrite history, so instead we do the next best thing: we compare a group of people who get the treatment with a group of similar people who don’t. It’s obvious, but nevertheless crucial, that the groups be comparable and representative of some population. If these conditions are met, then the results should be transferable to
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