15. During a study of 20 years 5 persons were followed to measure the occurrence of upper respiratory tract infection (URTI). As this infection can occur more than once, all disease events are included in this study. 1 person is lost to follow-up after 1.5 years. 2 persons died respectively after 10 and 15 years from a different cause. 1 person got the first respiratory tract infection after seven years and the second infection after 12 years of follow-up. Both infections take half a year of recovery. This person is followed-up until the end of the study. One person is followed-up the whole period without occurrence of disease. What is the incidence rate in this study?

Question

15.	During a study of 20 years 5 persons were followed to measure the occurrence of upper respiratory tract infection (URTI). As this infection can occur more than once, all disease events are included in this study. 1 person is lost to follow-up after 1.5 years. 2 persons died respectively after 10 and 15 years from a different cause. 1 person got the first respiratory tract infection after seven years and the second infection after 12 years of follow-up. Both infections take half a year of recovery. This person is followed-up until the end of the study. One person is followed-up the whole period without occurrence of disease. What is the incidence rate in this study?