A research study analyzed the health records of over 95,727 children. They divided the children at each age(2, 3, 4, or 5 years) into vaccinated for MMR and not vaccinated. 15,000 children were not vaccinated at age 2 and 8,000 remained not vaccinated at age 5. The autism rate for each group at each age was determined. The researchers found no association between the MMR vaccination and increased risk of autism spectrum disorder(ASD). This study is an

A retrospective cohort study

Explanation:

A retrospective cohort study is a type of observational epidemiologic study where the risk of disease is retrospectively (i.e., posteriorly) compared between an exposed group versus a non-exposed group. In a similar way to prospective cohort studies, a retrospective cohort study also categorizes individuals under study depending on if they were exposed or not to the factor of interest. However, in this type of experimental design, the study begins at a point in time after both exposure and outcome have already occurred. Thus, a retrospective study is conceived after some individuals have already developed the outcome (e.g., disease/disorder) of interest.