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In Reply: Drs Novikov and Kalter-Leibovici suggest that we include our instrumental variable of regional cardiac catheterization rate in the propensity score. The propensity score is used in analyses to remove confounding. We did not include this variable because it was not independently related to the outcome. Incorporating it into the standard models does not change the results for adjusted relative risk (multivariable model risk adjustment, 0.503; 95% CI, 0.495-0.513; propensity score risk adjustment, 0.517; 95% CI, 0.508-0.526; propensity-based matching, 0.539; 95% CI, 0.515-0.563). This is further evidence that regional catheterization rate is not related to patient health status, and since it is not a confounder, it is not required in the propensity score.

Regarding their comment that the difference between the propensity score and instrumental variable results relates to them being different analytical methods, this is actually the central point of our study. Instrumental variable analysis is conceptually different . . . [Full Text of this Article]

Therese A. Stukel, PhD
stukel@ices.on.ca
Institute for Clinical Evaluative Sciences
Toronto, Ontario

Elliott S. Fisher, MD, MPH
Center for the Evaluative Clinical Sciences
Dartmouth Medical School
Hanover, NH

David E. Wennberg, MD, MPH
Center for Outcomes Research and Evaluation
Maine Medical Center
Portland

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