In an occupational cohort, 5 workers were each followed for 4 years and another 5 workers were each followed for 2 years before the study ended. During follow-up, 3 cases of occupational asthma occurred. What is the incidence density of occupational asthma?

• A 0.05 per person-year
• B 0.3 per person-year
• C 0.15 per person-year
• D 0.1 per person-year ✓

Explanation

Total observation time equals (5 x 4) + (5 x 2) = 30 person-years. Incidence density is the number of new cases divided by person-time of observation, giving 3/30 = 0.1 per person-year. The commonest error is dividing cases by the number of workers (3/10), which gives cumulative incidence rather than incidence density and ignores the differing durations of follow-up.

Reference: Gordis Epidemiology, 6th ed.

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