A study follows 500 individuals for 2 years, contributing 900 person-years due to loss to follow-up; 15 new cases occur. Compute the incidence rate per 1,000 person-years.

• A. 11.1 per 1,000 person-years.
• B. 15.0 per 1,000 person-years.
• C. 16.7 per 1,000 person-years.
• D. 20.0 per 1,000 person-years.

Answer: (C)

Incidence rate uses person-time at risk to account for how long each person was actually observed. In this scenario, the total time at risk is 900 person-years because some participants were lost to follow-up, so the denominator isn’t the full 1000 person-years. With 15 new cases, the rate is 15 divided by 900, which equals 0.0167 new cases per person-year. Expressed per 1,000 person-years, multiply by 1000 to get 16.7. This demonstrates how losing participants to follow-up reduces the total person-time and, relative to the number of new cases, yields a higher rate than if everyone had been followed for the full 2 years (which would have given 15 per 1,000).