Confounding in Observational Studies Explained

Bikaramjit Mann*, 1, Evan Wood2, 3
1 Department of Medicine, University of Calgary, Canada
2 Department of Medicine, University of British Columbia, Canada
3 BC Centre for Excellence in HIV/AIDS, Canada

© 2012Mann et al..

open-access license: This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 International Public License (CC-BY 4.0), a copy of which is available at: ( This license permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

* Address correspondence to this author at the Department of Medicine, University of Calgary, Calgary, Alberta, T2N 2T9, Canada; Tel: 587-215- 3743; E-mail:


Practical and ethical constraints mean that many clinical and/or epidemiological questions cannot be answered through the implementation of a randomized controlled trial. Under these circumstances, observational studies are often required to assess relationships between certain exposures and disease outcomes. Unfortunately, observational studies are notoriously vulnerable to the effect of different types of “confounding,” a concept that is often a source of confusion among trainees, clinicians and users of health information. This article discusses the concept of confounding by way of examples and offers a simple guide for assessing the impact of is effects for learners of evidence-based medicine.

Keywords: Confounding, observational studies, critical appraisal, evidence-based medicine.