How much data is enough for multi-period did?

Multi-stage DID method needs to meet a certain amount of data and time span to ensure the reliability and stability of the research results. Specifically, the requirement for data mainly depends on the sample size, the number of treatment groups and control groups, and the regression model used, and can not give specific standard answers.

However, 9 time points and 7 processing groups may not be enough to support robust multiphase DID analysis, and there may be problems such as sample deviation and exogenous variables omission. In addition, the multi-period DID method also needs to test the assumptions of trend stationarity and parallel trend to ensure that these assumptions are met.

Therefore, it is suggested that sufficient samples and reasonable time span should be ensured according to the specific situation when conducting multi-stage DID research, and other hypotheses should be tested and analyzed for sensitivity to ensure the reliability and accuracy of the research results.