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Analysis of nonintervention studies: technical supplement.

Methods for analyzing data in nonintervention clinical studies are substantially different from those that are appropriate for randomized clinical trials. Although the latter methods are well known, the former are not. A systematic approach for dealing with statistical confounding in nonintervention research has been developed over the past 30 to 40 years, and the essence of this theory constitutes the contents of this article. An accompanying, less technical article explains the implications of these results for clinical research.