Abstract: In the past twenty years, causal inference has developed rapidly and gradually dominated the field of micrometrics. The paper first introduces the context of the emerging causal inference methods. Next, we discuss three preconditions to reach a causal conclusion, point out the major problems with making causal inference in the experimental and non experimental studies, and analyze the main causes and components of heterogeneous residual that commonly exist in the observation studies. Then, using cases of impact evaluation of small class teaching and new mechanism reform, the paper illustrates the basic principles and analyzes procedures of some quasi experimental methods, including regression discontinuity, instrumental variable, propensity score method and double difference. Finally, in response to the doubts about the internal validity of quasi experimental studies, we emphasize the importance of robustness and sensitivity test of the implicit hypothesis that hide behind quasi experimental methods.

Key words: causal inference, educational research, quasi experiment, heterogeneous residual, internal validity