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New paper: 'Unexplained residuals’ models for longitudinal data

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paper
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Arnold KF; Ellison GTH; Gadd SC; Textor J; Tennant PWG; Heppenstall A; Gilthorpe MS (2018) Adjustment for time-invariant and time-varying confounders in ‘unexplained residuals’ (UR) models for longitudinal data within a causal framework, and associated challenges http://eprints.whiterose.ac.uk/126041/, Statistical Methods in Medical Research

Abstract: ‘Unexplained residuals’ (UR) models have been used within lifecourse epidemiology to model an exposure measured longitudinally at several time points in relation to a distal outcome. It has been claimed that these models have several advantages, including: the ability to estimate multiple total causal effects in a single model, and additional insight into the effect on the outcome of greater-than-expected increases in the exposure compared to traditional regression methods. We evaluate these properties, and prove mathematically how adjustment for confounding variables must be made within this modelling framework. Importantly, we explicitly place UR models in a causal framework using directed acyclic graphs (DAGs). This allows for theoretical justification of appropriate confounder adjustment, and provides a framework for extending our results to more complex scenarios than those examined in this paper. We also discuss several interpretational issues relating to UR models within a causal framework. We argue that UR models offer no additional insights compared to traditional regression methods, and, in fact, are more challenging to implement; moreover, they artificially reduce estimated standard errors. Consequently, we conclude that UR models, if used, must be implemented with great care.