This study proposes a new Bayesian approach to infer binary treatment effects. The approach treats counterfactual untreated outcomes as missing observations and infers them by completing a matrix composed of realized and potential untreated outcomes using a data augmentation technique. We also develop a tailored prior that helps in the identification of parameters and induces the matrix of untreated outcomes to be approximately low rank. Posterior draws are simulated using a Markov Chain Monte Carlo sampler. While the proposed approach is similar to synthetic control methods and other related methods, it has several notable advantages. First, unlike synthetic control methods, the proposed approach does not require stringent assumptions. Second, in contrast to non-Bayesian approaches, the proposed method can quantify uncertainty about inferences in a straightforward and consistent manner. By means of a series of simulation studies, we show that our proposal has a better finite sample performance than that of the existing approaches.
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