Introducing incidence as a key concept in epidemiology
Incidence can be considered as the fundamental measure of occurrence as it determines the prevalence of a health outcome. The challenge for eye health is the lack of data on the incidence of eye diseases and visual
By the end of this module you will be able to:
1. Describe the definition of incidence
2. Understand how to calculate incidence
3. Relate to outcomes and exposures for eye diseases
4. Understand and reflect on the practical application of incidence
As you go through this module, consider the challenges in measuring incidence and think about how incidence is related to prevalence.
Feedback on the discussion on the epidemiology of visual impairment in The Gambia
In step the previous module, we examined some key questions about population surveys using the example of The Gambia survey. In particular, we asked you to consider 5 questions:
A. In order to plan this survey, where do you think the researcher obtained information on population data? The latest population census data would have been used.
B. Looking at the map of The Gambia, we can see that the capital city and towns are primarily located in the West of the country compared to the East. Is this relevant when trying to select a representative sample?
In order to obtain a good representation of the population, the sampling should ideally be selected from urban, peri-urban and rural communities. In this case, the selection for the samples would have been done also based on population density north and south of the river.
C. Why is it important to use a standardised examination protocol in an epidemiological survey? The survey team, use a standardized examination protocol so that variations between examiners is minimized, all case definitions are accurately followed and documented accurately by the study team.
In the long run, data from standardized protocols can also be easier to compare, observe trends and assess impact.
D. How do you think the researcher could make sure the protocol is used by all the staff carrying out the survey? It is essential, that prior to any fieldwork, that the team is trained to examine and document their findings in the same way, irrespective of the field setting and reduce bias.
E. If the population was 800,000, how many blind and visually impaired people were there in The Gambia in 1986?
Blind cases = 0.7% x 800,000 = 5,600
Low vision = 1.4% x 800,000 = 11,200
Total = 16,800