Robustness of ANCOVA in randomized trials with unequal randomization

Jonathan W. Bartlett

Research output: Contribution to journalArticlepeer-review

3 Citations (SciVal)
182 Downloads (Pure)


Randomized trials with continuous outcomes are often analyzed using ANCOVA, with adjustment for prognostic baseline covariates. The ANCOVA estimator of the treatment effect is consistent under arbitrary model misspecification. In an article recently published in the journal, Wang et al proved the model based variance estimator for the treatment effect is also consistent under outcome model misspecification, assuming the probability of randomization to each treatment is 1/2. In this reader reaction we derive explicit expressions which show that when randomization is unequal, the model based variance estimator can be biased upwards or downwards. In contrast, robust sandwich variance estimators can provide asymptotically valid inferences under arbitrary misspecification, even when randomization probabilities are not equal.
Original languageEnglish
Pages (from-to)1036-1038
Number of pages3
Issue number3
Early online date10 Dec 2019
Publication statusPublished - 10 Sept 2020


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