Lesson 1: Welcome to this module

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An introduction to cohort studies

The word “cohort” has its origin in Latin, “cohors” refers to warriors and the notion that as a group they proceed together in time. Cohort studies have several synonyms. They are known as prospective studies, follow-up studies, incidence studies and longitudinal surveys.

The cohort study design begins with the selection of a group of people with a common characteristic, such as location or year of birth, who are free of the outcome of interest (the disease). Researchers follow up this group (the cohort) over time, and methodically examine them at regular intervals to assess if the outcome of interest has occurred or not.

At the end of this module, you will be able to compare how many of the exposed cohort subjects have developed the disease versus the non-exposed subjects.

The key purpose of a cohort study is to determine the occurrence of a disease when exposed to a risk factor. This measure is known as incidence. Information on incidence is invaluable to a health system so as to enable resources to be allocated and appropriate public health interventions

In prospective cohort studies, subjects are followed up for a period of time and outcomes recorded. In retrospective cohort studies, both exposure and outcome have already occurred. This is dependent on previously collected data and therefore susceptible to the effects of bias.

Information of variable may also be unavailable or difficult to collect. We use diabetic retinopathy as our example of eye disease as we examine cohort studies.

By the end of this session you should be able to:

  1. Appreciate the challenge posed by diabetic retinopathy.
  2. Describe the identification of a population for a diabetic retinopathy cohort study.
  3. Identify the sources of error in cohort studies.
  4. Calculate the association between risk and outcome.
  5. Critically assess cohort studies and application in eye care programmes