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Reporting of Baseline Characteristics to Accompany Analysis by Instrumental Variables

MacKenzie, Todd A.a; O’Malley, A. Jamesa; Bekelis, Kimona,b

doi: 10.1097/EDE.0000000000000914

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.

From the aDartmouth College, Hanover, NH

bCatholic Health Systems of Long Island, Melville, NY.

Submitted January 16, 2018; accepted August 14, 2018.

Availability of data and code for replication: All code is available as specified in the article. The data are not available without permission from the Statewide Planning and Research Cooperative System (SPARCS) of the State of New York.

This work was partially supported by National Institutes of Health Grant 1RC4MH 092717-01 and through a Patient-Centered Outcomes Research Institute (PCORI) Award ME-1503-28261.

Disclosure: The authors report no conflicts of interest.

The authors report no conflicts of interest.

Supplemental digital content is available through direct URL citations in the HTML and PDF versions of this article (

Correspondence: Todd A. MacKenzie, Dartmouth College, Hanover, NH 03755. E-mail:

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