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A varicella outbreak ran through a boarding school from September to December. Which denominator gives the risk of illness among students?
ASusceptible students at the start of the period
BSusceptible students at the end of the period
CSusceptible students at the midpoint of the period
DAverage susceptible students across the period
Answer & Solution
Correct answer: A. Susceptible students at the start of the period
1. Risk is estimated by the incidence proportion, which is new cases divided by the population at risk.
2. That population is counted at the start of the observation period, here September.
3. Everyone counted in the denominator must have had the chance to appear in the numerator, which rules out counts taken after cases have already occurred.
4. Midpoint and average denominators belong to rates built on mid-interval population, not to a risk.
_Source: CDC Principles of Epidemiology in Public Health Practice (3rd ed., US federal, public domain), Lesson 3 "Measures of Risk", section More About Denominators_
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