A cohort study is designed to compare the incidence of a certain outcome between a group that has a certain characteristic and a group that lacks the characteristic. There are two basic ways of generating the study groups:

• The groups are selected based on this characteristic from the start.
• A defined population that was not selected based on this characteristic (i.e. a population that lives in a certain community) is separated into the study groups. This type of selecting a study population can allow investigators to observe multiple characteristics.