During the 5-year follow-up, 35 new cases occurred among 930 disease-free individuals at baseline. What is the 5-year cumulative incidence among the previously non-diseased?

• A. 10.5%
• B. 11.2%
• C. 3.7%
• D. 3.5%

Answer: (C)

Cumulative incidence assesses the probability that someone who is disease-free at the start will develop the disease during the defined period. Here, 35 new cases occurred among 930 individuals who were disease-free at baseline, so the five-year cumulative incidence is 35/930 = 0.0376, about 3.76%. In practice this is reported as roughly 3.7% (often rounded to 3.8% depending on rounding rules). This measure uses the baseline at-risk population, unlike incidence rate which would use person-time in the denominator, or prevalence which reflects existing cases at a single point in time.