Cohort study.

A cohort study is a type of analysis in which one group with something in common is studied in comparison with another similar group that does not have that same trait or characteristic. An example would be studying how many children with pets develop allergies in comparison to children who do not have pets. This type of study allows researchers to study several factors at one time. Cohort studies do present some problems, however, including the fact that they are expensive and time-consuming to conduct and are prone to being affected by people who change their behaviors and no longer fit the group criteria.