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American Journal of Epidemiology Advance Access published online on January 29, 2008
American Journal of Epidemiology, doi:10.1093/aje/kwm357
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American Journal of Epidemiology © The Author 2008. Published by the Johns Hopkins Bloomberg School of Public Health. All rights reserved. For permissions, please e-mail: journals.permissions@oxfordjournals.org.

Practice of Epidemiology

A Statistical Test for the Equality of Differently Adjusted Incidence Rate Ratios

Kurt Hoffmann *, Tobias Pischon{dagger}, Mandy Schulz, Matthias B. Schulze, Jennifer Ray and Heiner Boeing

From the Department of Epidemiology, German Institute of Human Nutrition Potsdam-Rehbrücke, Nuthetal, Germany

{dagger} 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 000, and the authors' response is published on page 000.

* Deceased.


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