Case ascertainment in stroke studies: the risk of selection bias

Objectives– The purpose was to compare the completeness of case ascertainment in two stroke registers, one local population‐based, the other a national quality register (Riks‐Stroke), and to examine if patient characteristics could be affected by selection bias. Material and methods– By the way of linking and matching computer files, the completeness of case ascertainment was evaluated. Results– In the local stroke incidence study 377 patients were included. Of them, 63% were reported to the hospital‐based national quality register. The case‐fatality was lower in the national register. A larger proportion of the patients in the national register appeared to have been treated in a stroke unit and undergone rehabilitation, and computerized tomography seemed to have been performed in a larger proportion. Conclusions– Because of selection bias, outcome data get skewed when case ascertainment does not embrace all stroke cases. A community‐based stroke register is the golden standard when measuring stroke incidence.