10 subjects are enrolled in a 5-year study. 4 subjects participate in the study for the entire period. 2 subjects are lost to follow-up after 3 years. 1 subject only participates for 1 year, and the remaining subjects die after 4 years.
What is the total person-years for this study?
Assume 3 subjects developed the condition under investigation.
Calculate the incidence density per 1,000 person-years.