Given a cohort of 1,000 disease-free individuals followed for 1 year with 20 new cases, calculate the cumulative incidence.

• A. 0.010 or 1%.
• B. 0.02 or 2%.
• C. 0.20 or 20%.
• D. 0.002 or 0.2%.

Answer: (B)

Cumulative incidence is the risk of developing the disease over a specified period, using the number at risk at the start as the denominator when follow-up is complete for everyone.

In this scenario, 1,000 disease-free individuals start, and 20 develop the disease over one year. So the cumulative incidence is 20 divided by 1,000, which equals 0.02 or 2%.

The other numbers would come from different counts: 10 cases would be 1%, 200 cases would be 20%, and 2 cases would be 0.2%.