To Adjust or Not to Adjust? When a "Confounder" Is Only Measured After Exposure

Rolf H H Groenwold*, Tom M Palmer, Kate M Tilling

*Corresponding author for this work

Research output: Contribution to journalArticle (Academic Journal)peer-review

30 Citations (Scopus)
86 Downloads (Pure)


Advice regarding the analysis of observational studies of exposure effects usually is against adjustment for factors that occur after the exposure, as they may be caused by the exposure (or mediate the effect of exposure on outcome), so potentially leading to collider stratification bias. However, such factors could also be caused by unmeasured confounding factors, in which case adjusting for them will also remove some of the bias due to confounding. We derive expressions for collider stratification bias when conditioning and confounding bias when not conditioning on the mediator, in the presence of unmeasured confounding (assuming that all associations are linear and there are no interactions). Using simulations, we show that generally neither the conditioned nor the unconditioned estimate is unbiased, and the trade-off between them depends on the magnitude of the effect of the exposure that is mediated relative to the effect of the unmeasured confounders and their relations with the mediator. We illustrate the use of the bias expressions via three examples: neuroticism and mortality (adjusting for the mediator appears the least biased option), glycated hemoglobin levels and systolic blood pressure (adjusting gives smaller bias), and literacy in primary school pupils (not adjusting gives smaller bias). Our formulae and simulations can inform quantitative bias analysis as well as analysis strategies for observational studies in which there is a potential for unmeasured confounding.
Original languageEnglish
Pages (from-to)194-201
Number of pages8
Issue number2
Early online date13 Jan 2021
Publication statusPublished - 1 Mar 2021

Bibliographical note

Publisher Copyright:
© 2021 Lippincott Williams and Wilkins. All rights reserved.


Dive into the research topics of 'To Adjust or Not to Adjust? When a "Confounder" Is Only Measured After Exposure'. Together they form a unique fingerprint.

Cite this