Confounding by indication: the case of calcium channel blockers.
OBJECTIVE
To review conceptual issues regarding confounding by indication in the context of studies of calcium channel blockers (CCBs).


METHODS
Review of literature, with special attention to two articles in the current issue.


RESULTS
Conflicting arguments about the presence of uncontrollable confounding by indication in studies of CCBs are reviewed and criticized. Studies with potential confounding by indication can benefit from appropriate analytic methods, including separating the effects of a drug taken at different times, sensitivity analysis for unmeasured confounders, and instrumental variables and G-estimation.


CONCLUSIONS
Whether confounding by indication accounts for observed associations is often difficult to determine; this is the case in studies of CCBs. When confounding by indication is suspected, a variety of methods to deal with it may be useful. Copyright (c) 2000 John Wiley & Sons, Ltd.
Joffe
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Division of Biostatistics, Department of Biostatistics and Epidemiology, Center for Clinical Epidemiology and Biostatistics, University of Pennsylvania School of Medicine, 602 Blockley Hall, 423 Guardian Drive, Philadelphia, PA 19104-6021, USA.