Department of Epidemiology, Swedish Institute for Infectious Disease Control, Tomtebodavägen 19A, 171 82 Solna, Sweden.
Am J Epidemiol. 2010 Mar 1;171(5):602-8. doi: 10.1093/aje/kwp432. Epub 2010 Jan 27.
Selection bias is a concern in cohort studies in which selection into the cohort is related to the studied outcome. An example is chronic infection with hepatitis C virus, where the initial infection may be asymptomatic for decades. This problem leads to selection of more severely ill individuals into registers of such infections. Cohort studies often adjust for this bias by introducing a time window between entry into the cohort and entry into the study. This paper describes and assesses a novel method to improve adjustment for this type of selection bias. The size of the time window is decided by calculating a standardized incidence ratio as a continuous function of the size of the time window. The resulting graph is used to decide on an appropriate window size. The method is evaluated by using the Swedish register of hepatitis C virus infections for 1990-2006. The complications studied were non-Hodgkin lymphoma and liver cancer. Selection bias differed for the studied outcomes, and a time window of a minimum of 2 months and 12 months, respectively, was judged to be appropriate. The novel method may have advantages compared with an interval-based method, especially in cohort studies with small numbers of events.
选择偏倚是队列研究中需要关注的问题，因为队列的选择与研究结果有关。例如，慢性丙型肝炎病毒感染，最初的感染可能在数十年内无症状。这个问题导致更严重的感染个体被选入此类感染的登记册中。队列研究通常通过在进入队列和进入研究之间引入时间窗来调整这种偏倚。本文描述并评估了一种改进这种选择偏倚调整的新方法。时间窗的大小通过将标准化发病比计算为时间窗大小的连续函数来确定。所得图形用于确定适当的窗口大小。该方法通过使用瑞典丙型肝炎病毒感染登记处 1990-2006 年的数据进行评估。所研究的并发症是非霍奇金淋巴瘤和肝癌。对于所研究的结果，选择偏倚不同，分别认为最小 2 个月和 12 个月的时间窗是合适的。与基于间隔的方法相比，新方法可能具有优势，尤其是在事件数量较少的队列研究中。
