biostudies-literatureUnknown56(5)Zeldow B<h4>Objective</h4>To define confounding bias in difference-in-difference studies and compare regression- and matching-based estimators designed to correct bias due to observed confounders.<h4>Data sources</h4>We simulated data from linear models that incorporated different confounding relationships: time-invariant covariates with a time-varying effect on the outcome, time-varying covariates with a constant effect on the outcome, and time-varying covariates with a time-varying effect on the outcome. We considered a simple setting that is common in the applied literature: treatment is introduced at a single time point and there is no unobserved treatment effect heterogeneity.<h4>Study design</h4>We compared the bias and root mean squared error of treatment effect estimates from six model speHealth services research932-941https://www.ebi.ac.uk/biostudies/studies/S-EPMC8522571biostudies-literatureConfounding and regression adjustment in difference-in-differences studies.PMC8522571Hatfield LAZeldow BfalseConfounding and regression adjustment in difference-in-differences studies.<h4>Objective</h4>To define confounding bias in difference-in-difference studies and compare regression- and matching-based estimators designed to correct bias due to observed confounders.<h4>Data sources</h4>We simulated data from linear models that incorporated different confounding relationships: time-invariant covariates with a time-varying effect on the outcome, time-varying covariates with a constant effect on the outcome, and time-varying covariates with a time-varying effect on the outcome. We considered a simple setting that is common in the applied literature: treatment is introduced at a single time point and there is no unobserved treatment effect heterogeneity.<h4>Study design</h4>We compared the bias and root mean squared error of treatment effect estimates from six model spe2021-01-01T00:00:00Z2021 Oct2025-04-04T10:21:26.129Z2025-04-04T10:21:26.129ZS-EPMC85225713397895610.1111/1475-6773.13666