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Why are cases falling outside the main body of an epidemic curve worth examining closely?
AThey always indicate errors in the recorded data
BThey set the case definition used in the study
CThey may carry an exposure pointing to the source
DThey fix the average incubation period exactly
Answer & Solution
Correct answer: C. They may carry an exposure pointing to the source
1. An early case may be the source itself, or someone exposed before the rest, such as a cook who tasted a dish.
2. A late case may be a secondary case, a long incubation, or someone who ate leftovers.
3. Either way, an outlier that belongs to the outbreak often has one easily identified exposure that points straight at the source.
4. Outliers are sometimes only miscoded data, so saying they always indicate error overstates a possibility as a rule.
5. The case definition is written before cases are plotted, so outliers cannot set it.
_Source: CDC Principles of Epidemiology in Public Health Practice, 3rd Edition, section Interpreting an Epidemic Curve_
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