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Selection bias in an instrumental variable analysis restricted to patients who were prescribed a subset of the treatment options. Z represents the proposed instrument; X represents the treatment the patient was prescribed (X = 1 if treatment 1, X = 2 if treatment 2, and X = 3 if neither treatment); U is a set of unmeasured confounders; and Y is the outcome. The box around X indicates restriction to patients prescribed X = 1 or X = 2.
