The other type of control data they used is the patient’s usual frequency of physical exertion over the past year. The idea is to compare the amount of time exposed to exertion and the amount of time not exposed to exertion over the past with the information of whether there was exertion during the hazard period. Since here "person-time" exposed to exertion is used, the design is rather like a "cohort" study than a traditional case-control study, and the analysis for this type of control data will also be more like a cohort study analysis.