A theoretical exploration of therapeutic monomania as a physician‐based instrumental variable

PURPOSE: To explore the utility of physician prescribing preference as an instrumental variable. METHODS: Expert (non‐systematic) review of relevant literature on the appropriate selection of instrumental variables and theoretical exploration of individual physician and physician group prescriber preference. RESULTS: An instrumental variable must satisfy three criteria: (1) It must predict the treatment received (strength of the instrument); (2) it cannot influence the outcome other that through the treatment received (exclusion restriction); and (3) it cannot be influenced by any factor that also influences the outcome (independence assumption). Arguments in favor of prescriber preference as an instrumental variable and suggestions for how to approach specific scenarios that may be encountered are offered. CONCLUSIONS: Prescriber preference, be it of individual physicians or groups of physicians, may, under the right conditions, be powerful instrumental variables. Empiric experimental data are required to determine the appropriateness of combining propensity matching and instrumental variable analysis.