British investigators conducted a study to compare measles-mumps-rubella (mmr) vaccine history among 1,294 children with pervasive development disorder (e. g., autism and asperger’s syndrome) and 4,469 children without such disorders. (they found no association.) this is an example of which type(s) of study?

Retrospective cohort study

Explanation:

Retrospective cohort studies are known as or referred to as the type or kind of observational research under which investigators tend to look back at historical data that is archived in order to examine if risk of the disease was different in between the patients who were exposed and non-exposed. Observational epidemiological designs  under which the risk of the disease is retrospectively compared in between the patients which are exposed and non-exposed.

no questions on whether they are allowed that right