You conduct a cohort study to examine whether using laptops is a risk factor for developing back pain among undergraduates. You enroll 50 UCM first-year undergraduates who use laptops (the exposed group) and 50 first-year undergraduates who use only a desktop computer (the unexposed group). You follow each group for two years and assess the incidence of severe back pain in each group at the end of the two years. Assume that no one has back pain at the beginning and no one drops out of the study. You find that 7 people in the exposed group report severe back pain at the end of the study. 3 people in the unexposed group report back pain.
Calculate the cumulative incidence of back pain in the exposed group.
A. 42.9% or 429 per 1,000
B. 86.0% or 860 per 1,000
C. 20.0% or 200 per 1,000
D. 6.0% or 60 per 1,000
E. 14.0% or 140 per 1,000