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3. During a study of 20 years five 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. - 1 person is lost to follow-up after 1.5 years. - 2 persons died respectively after 13 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