Hypothesis Generation:
Using Descriptive Epidemiology to Generate a Hypothesis Regarding the Source of Infection and Mode of Transmission
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In reality one begins to form hypotheses as soon as information about the outbreak begins to emerge. If you know what the disease is, your hypotheses will take into account its biology, what the reservoirs of disease are, how it is usually transmitted, its known risk factors, and its incubation period.
In addition, descriptive epidemiology attempts to identify similarities in person, time, or place that may suggest hypotheses regarding the cause of the outbreak.
For example, an epidemic curve illustrates the number of cases that occurred over time; it helps establish that there is an outbreak, and the shape of the curve may also suggest the nature of the source (e.g. isolated point source versus continuous common source).
Spot maps can be used to illustrate the residence or work place of each of the cases that may suggest the source, such as John Snow’s map of cholera cases clustered around the Broad St. pump.
Similarities in personal characteristics may also suggest the cause (e.g. common occupations, age, school attended, etc).
Descriptive epidemiology focuses entirely on similarities among the cases and does not establish the cause. Hypotheses that are generated need to be tested by analytic studies that compare two or more groups of people (e.g. cases and controls who did not get the disease).