Machine Learning, Causal Inference, Meta Learning, Instruments, Observational Studies.
2017). X-learner consists of three stages: Stage 1: Estimate the average outcome μ₀(x) and μ₁(x) where μ₀ (x) = E[ Y(0) | X=x ] and μ₁(x) = E[ Y(1) | X=x ] using arbitrary machine learning models. Stage 2: Impute the user-level treatment effects Dᵢ¹ for user i in the treatment group based on μ₀(x)