One of the commonly accepted merits of cohort studies (CSs) refers to the exposure precedes outcome superior to other observational designs. We use Directed Acyclic Graphs to construct a causal graph among research populations under CSs. We notice that the substitution of research population in place of a susceptible one can be used for effect estimation. Its correctness depends on the outcome-free status of the substituted population and the performance of both screening and diagnosis regarding the outcomes under study at baseline. The temporal precedence of exposure over outcome occurs theoretically, despite the opposite happens in realities. Correct effect estimate is affected by both the suitability of population substitution and the validities of outcome identification and exclusion.
暴露在发生时间上先于结局，是队列研究的优点之一，因此在因果推断上优于其他观察性设计。本文应用有向无环图（Directed Acyclic Graphs，DAGs）构建了现实队列研究中易感人群的因果结构后发现：现实的队列研究以研究人群替换易感人群进行因果效应的估计，人群的暴露与结局在时序关系上可互为先后，因果效应估计的准确性受到替换人群的易感性和基线调查时结局识别和排除有效性的影响。.
Keywords: Causality; Cohort study; Directed acyclic graphs; Susceptible population; Temporal precedence.