BMJ  2005;330:960-962 (23 April), doi:10.1136/bmj.330.7497.960

Education and debate

Reader's guide to critical appraisal of cohort studies: 2. Assessing potential for confounding

¹ Institute for Clinical Evaluative Sciences, Toronto, ON Canada, ² Department of Health Care Policy, Harvard Medical School, Boston, USA, ³ Department of Psychiatry, University of Toronto, ON, Canada, ⁴ Kunin-Lunenfeld Applied Research Unit, Baycrest Centre for Geriatric Care, Toronto, ON, Canada, ⁵ Department of Health Policy, Management and Evaluation, Faculty of Medicine, University of Toronto, Toronto, ON Canada

Although confounding is an important problem of cohort studies, its effects can be minimised to enable valid comparison

The first 150 words of the full text of this article appear below.

Introduction

In cohort studies, who does or does not receive an intervention is determined by practice patterns, personal choice, or policy decisions. This raises the possibility that the intervention and comparison groups may differ in characteristics that affect the study outcome, a problem called selection bias. If these characteristics have independent effects on the observed outcome in each group, they will create differences in outcomes between the groups apart from those related to the interventions being assessed. This effect is known as confounding.¹ In the first paper in the series we dealt with the design and use of cohort studies and how to identify selection bias.² This paper focuses on the definition and assessment of confounders.

What is a confounder?

For a characteristic to be a confounder in a particular study, it must meet two criteria.¹ The first is that it must be related to the outcome in terms of prognosis or susceptibility. For example, . . . [Full text of this article]

Has there been a systematic effort to identify and measure potential confounders?

Is there information on distribution of potential confounders between groups?

What methods are used to assess differences in distribution of potential confounders?

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Rapid Responses:

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Adding an operational definition to confounding

James M O'Brien
bmj.com, 26 Apr 2005 [Full text]

assessing differences in distribution

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