Answer:In a cohort study, individuals must be free of the disease of interest at the beginning of the study, allowing us to calculate the cumulative incidence or incidence rate.
Step-by-step explanation:
A cohort study is designed to investigate the development of a particular outcome or disease over time in a group of individuals who share a common characteristic or experience. In such a study, it is essential that participants are initially free of the disease of interest.
This criterion enables the calculation of the cumulative incidence or incidence rate. Cumulative incidence represents the proportion of individuals who develop the disease within a specified time period, while the incidence rate provides information on the rate at which new cases of the disease occur over time.
By starting with a disease-free cohort, researchers can accurately measure the occurrence of new cases and calculate the incidence, providing valuable insights into the risk and dynamics of the disease within the studied population. This foundational principle ensures the reliability of the study's findings and helps in understanding the natural history and determinants of the disease under investigation.