Show all work! No credit will be given when only answers are provided. All rates must be presented as X per 100, X per 1,000, etc.

4. During a study of 20 years 10 people are followed to measure the occurrence of upper respiratory tract infection. As this infection can occur more than once, all disease events are included in this study. Recovery time during an infection is half a year. What is the incidence density in this study (be sure to include units)? (2 points)

1 1 1 1 1 1 Person is lost to follow-up after 1.5 years Person is lost to follow-up after 3 years Person died after 7 years from a different cause Person died after 4.5 years from a different cause Person got an upper respiratory tract infection after 3 years and remained in the study until the end of 20 years Person got a respiratory tract infection after 5 years and was subsequently lost to follow-up at year 10 Person got the first respiratory infection after 2 years and a second respiratory infection during year 15. This person was followed until the end of the study Person is followed-up the whole period without occurrence of disease Person got an upper respiratory infection at year 17 and subsequently died from complications Person got an upper respiratory infection at year 3 and subsequently died from complications 1 1 1 1 1 1 1 1 1 1 Person is lost to follow-up after 1.5 years Person is lost to follow-up after 3 years Person died after 7 years from a different cause Person died after 4.5 years from a different cause Person got an upper respiratory tract infection after 3 years and remained in the study until the end of 20 years Person got a respiratory tract infection after 5 years and was subsequently lost to follow-up at year 10 Person got the first respiratory infection after 2 years and a second respiratory infection during year 15. This person was followed until the end of the study Person is followed-up the whole period without occurrence of disease Person got an upper respiratory infection at year 17 and subsequently died from complications Person got an upper respiratory infection at year 3 and subsequently died from complications 1 1 1 1