Lesson 6: Epidemiology in the Gambia

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The practical application of prevalence data

In this step we’re going to examine how to:

  • Identify good sources of population information for the denominator in prevalence calculations
  • Ensure that appropriate information is selected, and
  • Understand the relevance of prevalence data from surveys

Read the information below and think about the questions asked in each section.

Identifying good sources of population information

The Gambia is a small country, situated on the West Coast of Africa. In 1986 a population-based survey of blindness was conducted throughout the whole country. This was initiated by the Ministry of Health in The Gambia in collaboration with the International Centre for Eye Health and Prevention of Blindness programme at the World Health Organization.

The purpose of the survey was to provide baseline data on the prevalence of the major blinding eye diseases.

  • Question A. In order to plan this survey, where do you think the researcher obtained information on population data?
  • Question B. Now look at the map of The Gambia above. The capital city and towns are primarily located in the West compared to the East of the country. Is this relevant when trying to select a representative sample.

Ensuring that appropriate information is selected

The sampling strategy involved multi-stage random sampling ( will be explained later in the course), stratified by districts north and south of the river, by urban, peri-urban and rural communities, and by settlement sizes of less than and more than 300 persons. A total sample of 8,000 people was needed.

The researchers collected data was collected using a standardized eye examination protocol which included visual acuity testing and an eye examination.

• Question C. Why is it important to use a standardised examination protocol in an epidemiological survey?
• Question D. How do you think the researcher could make sure the protocol is used by all the staff carrying out the survey?

Understanding the relevance of prevalence data

Coverage of the survey was 94%. The data estimated the prevalence of blindness less than 3/60 was 0.7%. The prevalence of low vision was found to be 1.4%.

  • Question E. If the population was 800,000, how many blind and visually impaired people were there in The Gambia in 1986?

Discussion points

  1. Can you think of some of the reasons why retrospective data collection might have limitations?
  2. Where would you look for sources of population information in your setting?
  3. Answer questions A to E above (we share feedback on these questions at the start of session 2).