Some readers of my books and articles have suggested that the rule governing data aggregation and separation rests simply on the temporal precedence of the treatment and the “lurking third variable.” They argue that we should aggregate the data in the case of blood pressure because the blood pressure measurement comes after the patient takes the drug, but we should stratify the data in the case of gender because it is determined before the patient takes the drug. While this rule will work in a great many cases, it is not foolproof. A simple case is that of M-bias (Game 4 in Chapter 4). Here B
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