In a study conducted at the University of Colorado, J. Ruttenber and colleagues studied people who had worked at the Rocky Flats nuclear weapons production facility near Denver, Colorado. They studied a group of workers who had contracted lung cancer, and another group who had not contracted lung cancer. They looked back at plant records to determine the amount of radiation exposure for each worker.

The purpose of the study was to determine whether the people with lung cancer had been exposed to higher levels of radiation than those who had not gotten lung cancer.

a. Was this a cohort study or a case-control study?

b. Was the study prospective, cross-sectional, or retrospective?