Published online by Cambridge University Press: 01 January 2025
Covariate-adjusted treatment effects are commonly estimated in non-randomized studies. It has been shown that measurement error in covariates can bias treatment effect estimates when not appropriately accounted for. So far, these delineations primarily assumed a true data generating model that included just one single covariate. It is, however, more plausible that the true model consists of more than one covariate. We evaluate when a further covariate may reduce bias due to measurement error in another covariate and in which cases it is not recommended to include a further covariate. We analytically derive the amount of bias related to the fallible covariate’s reliability and systematically disentangle bias compensation and amplification due to an additional covariate. With a fallible covariate, it is not always beneficial to include an additional covariate for adjustment, as the additional covariate can extensively increase the bias. The mechanisms for an increased bias due to an additional covariate can be complex, even in a simple setting of just two covariates. A high reliability of the fallible covariate or a high correlation between the covariates cannot in general prevent from substantial bias. We show distorting effects of a fallible covariate in an empirical example and discuss adjustment for latent covariates as a possible solution.
Electronic supplementary material The online version of this article (https://doi.org/10.1007/s11336-019-09665-6) contains supplementary material, which is available to authorized users.
We thank Peter M. Steiner and Rolf Steyer for valuable discussions as well as Renate Soellner and Jens Eisermann who provided the empirical data. Furthermore, we thank the anonymous reviewers for insightful comments and constructive suggestions.