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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.