Suppose we are doing an observational study and have collected data on Smoking, Tar, and Cancer for each of the participants. Unfortunately, we cannot collect data on the Smoking Gene because we do not know whether such a gene exists. Lacking data on the confounding variable, we cannot block the back-door path Smoking Smoking Gene Cancer. Thus we cannot use back-door adjustment to control for the effect of the confounder. So we must look for another way. Instead of going in the back door, we can go in the front door! In this case, the front door is the direct causal path Smoking Tar Cancer,
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