Letters Section Editor: Robert M. Golub, MD, Senior Editor.
In Reply: The letter by Dr Stukel and colleagues helps to clarify their use of instrumental variable analysis. We do not, however, find their points convincing enough to resolve problems and concerns.
The claim that their instrumental variable estimates are unbiased seems to be a definition and not a validation. Their point about including regional catheterization rates in a propensity analysis, and that the variable “regional catheterization rates” was not related to patient health status, does not seem to take into account that regions with better health facilities and better trained physicians usually have better health outcomes, and these are the regions that are more likely to have higher rates of catheterization.
D’Agostino RB, D’Agostino RB. Using Observational Data to Estimate Treatment Effects—Reply. JAMA. 2007;297(19):2078-2079. doi:10.1001/jama.297.19.2079