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Epidemiological Methods in Studies of Symptoms in Advanced Disease
Author Bios
Why Study Advanced Disease?
Why Epidemiology?
Incidence and Prevalence
Currently selected selection: Using Incidence and Prevalence
Definition of a Case
Defining Time, Place, Person
Types of Study Design
Cross-Sectional Studies
Longitudinal Studies
Selection Bias
Measurement Bias
Presenting and Interpreting Results
Practical Example
Calculating Prevalence

Chapter 19: Epidemiological Methods in Studies of Symptoms in Advanced Disease: Using Incidence and Prevalence

Incidence and prevalence are closely related. Prevalence (the proportion of a population with a problem at a designated time) depends on both the incidence (the rate of new problem during a period of time) and the duration of the problem as is illustrated by the following cartoon.

Figure 4.1: Illustration of Incidence and Prevalence
Cartoon of two individuals standing in front of a puddle. Text says, “…just what we needed, a puddle too big to walk around.”  Incidence: the rain arriving; prevalence: the water in the puddle new and old; period prevalence: during a period; point prevalence: at one point in time. The water draining away into the soil or into drains reduces the puddle (prevalence), just as recovery or death reduces the number of patients with a problem.

A problem with a long duration will have a high probability of being encountered at that designated time when prevalence is estimated.

For example, the prevalence of chronic disease in a general population is growing because of both the higher incidence of chronic diseases (there are more cases each year than in the past) and the longer survival of patients with chronic disease resulting from modern treatments. If death rates drop, then the time that patients live with chronic disease grows and even a low incidence will produce a high prevalence.

Under specific assumptions, the inter-relationship between incidence and prevalence can be expressed by this formula: Prevalence = incidence x average duration.

Incidence is often used to describe the number of new cases of a particular disease, for example, lung cancer, breast cancer, heart disease. Incidence is useful in understanding how commonly a disease or problem occurs over a period of time, which can be used to estimate one's risk of getting the disease.

Prevalence is a more relevant measure than incidence when assessing the impact of a problem within a community and to assess the subsequent needs. Although data on prevalence are easier to collect than incidence data, prevalence should be used with caution in evaluating associations between variables. In studies of prevalence it is very difficult to distinguish the effects of factors affecting the occurrence of disease from the effects of factors that increase survival (or duration of the problem). This difficulty often leads to incorrect conclusions.

For example, imagine you are planning health services in a community and you want to know how many services are need to care for patients with motor neuron disease. The incidence is relatively low. But if only incidence was used to determine service needs, the need might be underestimated. The mean survival for patients with motor neuron disease is three to five years. During many of these years patients experience increasing symptoms and problems. Thus prevalence is more useful to assess the need for services.


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