If patients who were identical in every way were put into two groups, and the groups were treated differently, he wrote, we would know the treatment caused any difference in outcome. It seems simple but is impossible in practice because no two people are exactly alike, not even identical twins, so the experiment will be confounded by the differences among test subjects. The solution lay in statistics: Randomly assigning people to one group or the other would mean whatever differences there are among them should balance out if enough people participated in the experiment. Then we can
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