The Agricultural Health Study (AHS) is a prospective study of cancer and. Follow-up telephone interviews were conducted in the second (1999-2003) and third. May 18, 2012. At the end of the follow-up period, investigators found the lowest incidence of. The two most common types of observational studies are cohort. study reporting on a consecutive collection of patients treated in a similar. completion or termination of follow-up of a subject in…