Bias Magnification in Ecologic Studies: A Methodological Investigation


Show simple item record Webster, Thomas F en_US 2011-12-29T22:49:14Z 2011-12-29T22:49:14Z 2007 en_US 2007-7-5 en_US
dc.identifier.citation Webster, Thomas F. "Bias magnification in ecologic studies: a methodological investigation" Environmental Health 6:17. (2007) en_US
dc.identifier.issn 1476-069X en_US
dc.description.abstract BACKGROUND: As ecologic studies are often inexpensive to conduct, consideration of the magnitude and direction of ecologic biases may be useful in both study design and sensitivity analysis of results. This paper examines three types of ecologic bias: confounding by group, effect measure modification by group, and non-differential exposure misclassification. METHODS: Bias of the risk difference on the individual and ecologic levels are compared using two-by-two tables, simple equations, and risk diagrams. Risk diagrams provide a convenient way to simultaneously display information from both levels. RESULTS: Confounding by group and effect measure modification by group act in the same direction on the individual and group levels, but have larger impact on the latter. The reduction in exposure variance caused by aggregation magnifies the individual level bias due to ignoring groups. For some studies, the magnification factor can be calculated from the ecologic data alone. Small magnification factors indicate little bias beyond that occurring at the individual level. Aggregation is also responsible for the different impacts of non-differential exposure misclassification on individual and ecologic studies. CONCLUSION: The analytical tools developed here are useful in analyzing ecologic bias. The concept of bias magnification may be helpful in designing ecologic studies and performing sensitivity analysis of their results. en_US
dc.description.sponsorship National Institute of Environmental Health Sciences (5P42ES007381); National Institutes of Health en_US
dc.language.iso en en_US
dc.publisher BioMed Central en_US
dc.rights Copyright 2007 Webster; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution 2.0 License (, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. en_US
dc.rights.uri en_US
dc.title Bias Magnification in Ecologic Studies: A Methodological Investigation en_US
dc.type article en_US
dc.identifier.doi 10.1186/1476-069X-6-17 en_US
dc.identifier.pubmedid 17615079 en_US
dc.identifier.pmcid 1963324 en_US

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