Drug manufactures of Drug X, which was released on the market 5 years ago to treat Crohn’s disease, have noticed that many individuals treated with the drug have developed drug-induced lupus. A cohort...

Drug manufactures of Drug X, which was released on the market 5 years ago to treat Crohn’s disease, have noticed that many individuals treated with the drug have developed drug-induced lupus. A cohort study was conducted for post-marketing surveillance. It has documented that 6 individuals have developed drug-induced lupus over the course of the past 5 years. If 1 individual developed the disease at the 1-year mark, 3 developed the disease at the 2-year mark, 1 developed the disease at the 3-year mark, and the final individual developed the disease at the 4-year mark, what is the incidence rate of drug-induced lupus?