Study design

Researchers investigated the direct impact of hip fracture on mortality in a cohort of individuals aged over 65 years living in a geographically defined area in western Sydney, Australia.1 At baseline, no one in the cohort had ever experienced a hip fracture. The cohort was followed prospectively over an 18 month study period, during which 211 cases of hip fracture occurred. A total of 201 controls who had not experienced a hip fracture were selected at random from the same cohort. When identified, both cases with hip fracture and controls were subsequently followed for a further year. One year mortality among individuals with hip fracture was 21.7%, compared with 4.7% for controls. Which one of the following terms best describes the above study design?