In a village of 10,000 people, 200 new cases of tuberculosis were diagnosed over one year. The total person-years of observation was 9,800. At the start of the year, 50 people already had TB. Which of the following best represents the incidence rate of TB in this village?
- A 250 per 10,000 person-years
- B 200 per 9,800 person-years ✓
- C 200 per 10,000 population
- D 50 per 10,000 population
Correct answer: B. 200 per 9,800 person-years
Explanation
Incidence rate (incidence density) is calculated as the number of new cases divided by the total person-time at risk, expressed as cases per person-years. Here, 200 new cases occurred over 9,800 person-years of observation, giving an incidence rate of 200/9,800 person-years. The 50 prevalent cases at the start are excluded from the denominator since they were not at risk of becoming new cases.
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
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