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Cohort studies in sexual health
  1. Caroline A Sabin,
  2. Andrew N Phillips
  1. Royal Free Centre for HIV Medicine, London
  1. Dr C A Sabin, Department of Primary Care and Population Sciences, Royal Free and University College Medical School, Rowland Hill Street, London NW3 2PF, UK c.sabin{at}pcps.ucl.ac.uk

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J M Stephenson, A Babiker

Introduction

Examples of cohort studies in the sexual health literature are now increasingly common. The purpose of this article is to provide an overview of the design and analysis of cohort studies, along with a discussion of some of the potential problems with these studies which may be particularly relevant for cohorts which study factors related to sexual health. A comprehensive guide to the design and analysis of cohort studies can be found in the book by Breslow and Day.1

What is a cohort study?

A cohort study is a type of study in which a group of individuals is followed through time. Although cohort studies are frequently referred to in the literature, one of the distinguishing features of an epidemiological cohort study (as opposed to the literal meaning of the word “cohort” as a “group or gathering” of individuals), is that it is longitudinal—that is, there is some follow up of the individuals recruited to the study. Thus, many so called cohort studies are simply cross sectional studies, where a group of individuals is recruited for a one-off assessment, and are not followed any further. We will not consider these studies in this article.

The usual aim of a cohort study is to assess the relation between a specific event or disease outcome and exposure to one or more potential “risk” factors. The information on whether each individual has been exposed to these risk factors is usually collected at entry to the cohort, and the individuals recruited to the study are then monitored over time to see whether or not the event occurs. We may be interested in either the incidence of the event (that is, the rate of new events over a given time period), the average time until the event occurs, or the cumulative probability of …

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