Published 7 January 2009, doi:10.1136/bmj.a3191
Cite this as: BMJ 2009;338:a3191

Endgames

Statistical question

Study design

John Fletcher, clinical epidemiologist

1 BMJ, London WC1H 9JR

jfletcher{at}bmj.com

A neurologist wanted to study the risk of stroke and transient ischaemic attack in people with a diagnosis of migraine. From her headache clinic she selected cases of migraine and controls without migraine. She collected stroke and transient ischaemic attack outcomes by writing to all subjects and their general practitioners five years later. Which study design best describes what was done?

a Ecological study
b Cohort study
c Clinical trial
d Case-control study

Show Answers

Answer

b—Cohort study. The case control design looks at the end of a disease or process. Patients are recruited into the study in two groups. One group, the cases, has already had an outcome and the other group, the controls, has not. The question is then posed as to what these two groups of people did in the past.

Despite the wording in this study "cases" of migraine and "controls" without migraine are in fact the beginning of a study looking forward over time to the final outcome of stroke or transient ischaemic attack. This makes the study a cohort study. Migraine is examined as a potential risk factor. In this study migraine is not the outcome but the exposure.

An ecological study might compare hospital stroke admissions data with prescriptions of migraine medication on a county by county basis throughout the United Kingdom.

A clinical trial is an experiment. The clinical investigator decides what intervention each person will receive and then follows each person over time to observe the effects. As described, the above study does not involve any such intervention.

Cite this as: BMJ 2009;338:a3191


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