Direct Assessment of Completeness of Ascertainment in a Stroke Incidence Study

Background and Purpose— Validity of comparisons of stroke incidence between studies or time periods depends on the completeness of ascertainment. Ascertainment cannot be reliably assessed indirectly by statistical methods, such as capture–recapture. We report the first use of direct methods to determine the completeness of different ascertainment strategies in a population-based stroke incidence study (Oxford Vascular Study). Methods— We assessed completeness of 2 different ascertainment strategies: the core methods common to most previous incidence studies and core plus supplementary methods used in some studies (including access to carotid and brain imaging referrals and assessment of patients referred as “transient ischemic attack” or “recurrent stroke”). We assessed completeness of ascertainment in 2 ways. First, we searched anonymized primary care electronic patient records of the whole study population (n=90 542). Second, we interviewed and followed-up a high-risk subset of our study population: all...