Before revision:  To improve the estimation efficiency, we propose a new method that combines experimental and observational data by leveraging covariates that satisfy the first-stage relevance condition in the observational data .
After revision:  To improve the estimation efficiency, we propose a new method that combines experimental and observational datasets when 1) units from these two datasets are sampled from the same population and 2) some characteristics of these units are observed. I show that if these characteristics can partially explain treatment assignment in the observational data .