Reporting of Baseline Characteristics to Accompany Analysis by Instrumental Variables

A property and criticism of instrumental variable (IV) estimators is that they estimate the exposure effect in the compliers, individuals whose exposure is influenced by the instrument. It is conceivable that the exposure effect is different in individuals whose exposure is not influenced by the instrument. For that reason, it is useful to know who the compliers are. We present methods for reporting averages and other statistics and comparing them between the compliers and noncompliers, which are applicable to scenarios in which the instrument and exposure are fixed in time and dichotomous. The methods are illustrated in a comparison of outcomes between teaching and nonteaching hospitals in patients undergoing brain or spinal surgery.