In a prospective occupational cohort, workers contribute a total of 5,000 person-years of observation and 10 new cases of silicosis occur during follow-up. Which measure of disease frequency is most appropriate, and what is its value?
- A Incidence density, 2 per 1,000 person-years ✓
- B Cumulative incidence, 0.2%
- C Incidence density, 0.2 per 1,000 person-years
- D Point prevalence, 2%
Correct answer: A. Incidence density, 2 per 1,000 person-years
Explanation
When follow-up duration varies between participants, as happens with staggered entry and losses to follow-up, the correct denominator is summed person-time, giving incidence density: 10 divided by 5,000 person-years equals 2 per 1,000 person-years. Cumulative incidence requires a fixed denominator of people observed for the same period and cannot be computed here. Prevalence measures existing cases at one point in time and is irrelevant in an incident-cohort analysis.
Reference: Park's Textbook of Preventive and Social Medicine, 26th ed.
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