Robins and Greenland set out to express their conception of confounding in terms of potential outcomes. They partitioned the population into four types of individuals: doomed, causative, preventive, and immune. The language is suggestive, so let’s think of the treatment X as a flu vaccination and the outcome Y as coming down with flu. The doomed people are those for whom the vaccine doesn’t work; they will get flu whether they get the vaccine or not. The causative group (which may be nonexistent) includes those for whom the vaccine actually causes the disease. The preventive group consists of
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