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| EPI.2-4: The Link Between Prevalence and Incidence Measures |
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| EPI.2-4: The Link Between Prevalence and Incidence Measures |
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OBJECTIVES |
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At the end of this session you should understand the link between prevalence and incidence measures, and which measure is most useful in different circumstances |
The magnitude of disease prevalence, P obviously depends on incidence,I, since a greater rate of occurrence of new cases will tend to increase the number of existing cases; but it also depends on the duration, D of the disease. i.e. :
P µ I x D
Thus, a change in prevalence may be an effect either of a change in incidence or a change in duration of the disease. The duration in turn depends upon the time it takes to get well or the survival time with the disease.
In epidemiologic studies where the aim is to explore causal theories or to evaluate effects of preventive means, the interest is focused on the rate of flow of cases from the disease-free state to the diseased state. The relevant measure of disease occurrence, therefore, is incidence.
Measures of prevalence may be relevant in connection with the planning of health services or in assessing the need for medical care in a population. Occasionally, the choice between incidence or prevalence is made for pragmatic reasons.
For example, studies of commonly occurring chronic diseases such as high blood pressure, in which the point of transition from non-diseased to diseased occurs gradually, and the definition of disease is arbitrary, generally employ prevalence measures, whereas studies of mesothelioma or pesticide poisoning cases, which are hard to find at any specific point in time, generally use incidence measures
