Before revision:  A large set of covariates that admits an unknown, possibly nonlinear factor structure is exploited to control for the latent confounders. The key building block is a local principal subspace approximation procedure that combines K-nearest neighbors matching and principal component analysis.
After revision:  A large set of covariates that admits an unknown, possibly nonlinear factor structure . The main building block is a local principal subspace approximation procedure that combines K-nearest neighbors matching and principal component analysis.