Rolf H.H. Groenwold|Tom Palmer|Kate Tilling
To Adjust or Not to Adjust? When a “Confounder” Is Only Measured After Exposure
Leiden University Medical Center|Lancaster University|University of Bristol|University of Bristol
advice regarding analysis observational studies exposure effects usually adjustment factors occur exposure may caused exposure mediate effect exposure outcome potentially leading collider stratification bias however factors could also caused unmeasured confounding factors case adjusting also remove bias due confounding derive expressions collider stratification bias conditioning confounding bias conditioning mediator presence unmeasured confounding assuming associations linear interactions using simulations show generally neither conditioned unconditioned estimate unbiased tradeoff depends magnitude effect exposure mediated relative effect unmeasured confounders relations mediator illustrate use bias expressions via three examples neuroticism mortality adjusting mediator appears least biased option glycated hemoglobin levels systolic blood pressure adjusting gives smaller bias literacy primary school pupils adjusting gives smaller bias formulae simulations inform quantitative bias analysis well analysis strategies observational studies potential unmeasured confounding