A researcher follows a dynamic cohort of workers in an asbestos factory. During the first year, 100 workers contribute 90 person-years before 10 develop mesothelioma. During the second year, 90 workers contribute 85 person-years and 9 develop mesothelioma. What is the incidence density of mesothelioma per 100 person-years?
- A Approximately 19.0
- B Approximately 10.5 ✓
- C Approximately 11.1
- D Approximately 21.2
Correct answer: B. Approximately 10.5
Explanation
Incidence density equals total new cases divided by total person-time of observation. Total cases are 10 plus 9, giving 19. Total person-time is 90 plus 85, giving 175 person-years. The rate is therefore 19 divided by 175 multiplied by 100, which is approximately 10.5 per 100 person-years. Option C comes from wrongly using 171 person-years, and option A forgets to divide by two years of pooled follow-up.
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
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