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Research and Reporting Methods |

Distinguishing Case Series From Cohort Studies

Olaf M. Dekkers, MD, PhD, MA, MSc; Matthias Egger, MD, MSc; Douglas G. Altman, DSc; and Jan P. Vandenbroucke, MD, PhD
[+] Article and Author Information

From Leiden University Medical Center, Leiden, the Netherlands; University of Bern, Bern, Switzerland; and Center for Statistics in Medicine, University of Oxford, Oxford, United Kingdom.


Acknowledgment: The authors thank Geert van der Heijden for his critical discussion of our article.

Potential Conflicts of Interest: Disclosures can be viewed at www.acponline.org/authors/icmje/ConflictOfInterestForms.do?msNum=M11-1544.

Requests for Single Reprints: Olaf M. Dekkers, MD, PhD, MA, MSc, Department of Clinical Epidemiology, Leiden University Medical Center, Box 9600, 2300 RC Leiden, the Netherlands; e-mail, o.m.dekkers@lumc.nl.

Current Author Addresses: Drs. Dekkers and Vandenbroucke: Department of Clinical Epidemiology, Leiden University Medical Center, Box 9600, 2300 RC Leiden, the Netherlands.

Dr. Egger: University of Bern, Institute of Social and Preventive Medicine, Finkenhubelweg 11, CH-3012 Bern, Switzerland.

Dr. Altman: Centre for Statistics in Medicine, Wolfson College Annexe, Linton Road, Oxford OX2 6UD, United Kingdom.

Author Contributions: Conception and design: O.M. Dekkers, M. Egger, D.G. Altman, J.P. Vandenbroucke.

Drafting of the article: O.M. Dekkers, M. Egger, D.G. Altman, J.P. Vandenbroucke.

Critical revision of the article for important intellectual content: O.M. Dekkers, M. Egger, D.G. Altman, J.P. Vandenbroucke.

Final approval of the article: O.M. Dekkers, M. Egger, D.G. Altman, J.P. Vandenbroucke.

Statistical expertise: D.G. Altman.


Ann Intern Med. 2012;156(1_Part_1):37-40. doi:10.7326/0003-4819-156-1-201201030-00006
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Case series are a commonly reported study design, but the label “case series” is used inconsistently and sometimes incorrectly. Mislabeling impairs the appropriate indexing and sorting of evidence.

This article tries to clarify the concept of case series and proposes a way to distinguish them from cohort studies. In a cohort study, patients are sampled on the basis of exposure and are followed over time, and the occurrence of outcomes is assessed. A cohort study may include a comparison group, although this is not a necessary feature. A case series may be a study that samples patients with both a specific outcome and a specific exposure, or one that samples patients with a specific outcome and includes patients regardless of whether they have specific exposures. Whereas a cohort study, in principle, enables the calculation of an absolute risk or a rate for the outcome, such a calculation is not possible in a case series.

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