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Which assumption weakens person-time denominators in studies of chronic disease?
AThat every participant is followed for the same span
BThat no participant is ever lost to follow-up
CThat disease probability stays constant over time
DThat population size is measured at mid-interval
Answer & Solution
Correct answer: C. That disease probability stays constant over time
1. Person-time treats 10 people followed for one year as equivalent to one person followed for 10 years.
2. That equivalence only holds if the probability of disease is the same in every year of follow-up.
3. Risk of many chronic diseases climbs with age, so the assumption often fails.
4. Handling unequal follow-up is a strength of person-time, not a weakness, so the first option is backwards.
5. Person-time is built precisely to absorb losses to follow-up, and mid-interval population belongs to rates computed from vital statistics instead.
_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 incidence rates_
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