American Journal of Epidemiology Advance Access originally published online on January 29, 2008
American Journal of Epidemiology 2008 167(5):517-522; doi:10.1093/aje/kwm357
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PRACTICE OF EPIDEMIOLOGY |
A Statistical Test for the Equality of Differently Adjusted Incidence Rate Ratios
From the Department of Epidemiology, German Institute of Human Nutrition Potsdam-Rehbrücke, Nuthetal, Germany
Correspondence to Dr. Tobias Pischon, Department of Epidemiology, German Institute of Human Nutrition Potsdam-Rehbrücke, Arthur-Scheunert-Allee 114–116, 14558 Nuthetal, Germany (e-mail: pischon{at}dife.de).
Received for publication July 26, 2006. Accepted for publication January 5, 2007.
An incidence rate ratio (IRR) is a meaningful effect measure in epidemiology if it is adjusted for all important confounders. For evaluation of the impact of adjustment, adjusted IRRs should be compared with crude IRRs. The aim of this methodological study was to present a statistical approach for testing the equality of adjusted and crude IRRs and to derive a confidence interval for the ratio of the two IRRs. The method can be extended to compare two differently adjusted IRRs and, thus, to evaluate the effect of additional adjustment. The method runs immediately on existing software. To illustrate the application of this approach, the authors studied adjusted IRRs for two risk factors of type 2 diabetes using data from the European Prospective Investigation into Cancer and Nutrition-Potsdam Study from 2005. The statistical method described may be helpful as an additional tool for analyzing epidemiologic cohort data and for interpreting results obtained from Cox regression models with adjustment for different covariates.
cohort studies; confounding; diabetes mellitus, type 2; epidemiologic methods; proportional hazards models; statistics
Abbreviations: IRR, incidence rate ratio
Editor's note: An invited commentary on this article appears on page 523, and the authors' response is published on page 530.
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Related articles in Am. J. Epidemiol.:
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Am. J. Epidemiol. 2008 167: 523-529.[Abstract] [Full Text] - Pischon et al. Respond to "Variable Selection versus Shrinkage in Control of Confounders"
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Am. J. Epidemiol. 2008 167: 530-531.[Extract] [Full Text]
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  S. Greenland Invited Commentary: Variable Selection versus Shrinkage in the Control of Multiple Confounders Am. J. Epidemiol., March 1, 2008; 167(5): 523 - 529. [Abstract] [Full Text] [PDF]
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T. Pischon, M. B. Schulze, D. Drogan, and H. Boeing Pischon et al. Respond to "Variable Selection versus Shrinkage in Control of Confounders" Am. J. Epidemiol., March 1, 2008; 167(5): 530 - 531. [Full Text] [PDF]
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