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Prevalence and incidence differ chiefly in which part of the fraction?
AThe numerator, since prevalence counts existing cases
BThe multiplier, since prevalence always uses 100,000
CThe denominator, since prevalence uses person-time
DThe interval, since incidence covers a single day
Answer & Solution
Correct answer: A. The numerator, since prevalence counts existing cases
1. The numerator of incidence counts only cases whose illness began during the specified interval.
2. The numerator of prevalence counts everyone ill during the interval, whenever the illness began.
3. That is the key difference between the two measures.
4. Prevalence uses a multiplier of 1 or 100 for common attributes and larger values only for rare ones, so the multiplier is not fixed.
5. Person-time belongs to the incidence rate, not prevalence, and neither measure is restricted to a single day.
_Source: CDC Principles of Epidemiology in Public Health Practice (3rd ed., US federal, public domain), Lesson 3 "Measures of Risk", section Properties and uses of prevalence_
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