A Proposed Method to Adjust for Selection Bias in Cohort Studies

摘要

Selection bias is a concern in cohort studies in which selection into the cohort is related to the studied outcome.nAn example is chronic infection with hepatitis C virus, where the initial infection may be asymptomatic for decades.nThis problem leads to selection of more severely ill individuals into registers of such infections. Cohort studies oftennadjust for this bias by introducing a time window between entry into the cohort and entry into the study. This paperndescribes and assesses a novel method to improve adjustment for this type of selection bias. The size of the timenwindow is decided by calculating a standardized incidence ratio as a continuous function of the size of the timenwindow. The resulting graph is used to decide on an appropriate window size. The method is evaluated by usingnthe Swedish register of hepatitis C virus infections for 1990–2006. The complications studied were non-Hodgkinnlymphoma and liver cancer. Selection bias differed for the studied outcomes, and a time window of a minimum ofn2 months and 12 months, respectively, was judged to be appropriate. The novel method may have advantagesncompared with an interval-based method, especially in cohort studies with small numbers of events.