Abstract:Sample size is one of important factors determining the results of a clinical trial, and the choice of sample size estimation methods is related to the specific design of clinical trials. On clinical safety reevaluation of postmarketing Chinese medicine,we often design cohort study, case-control study,nested case-control study or hospital centralized monitoring,which need large sample sizes. In evaluating the effectiveness of postmarketing Chinese medicine widely applied in the large population, pragmatic randomized controlled trial(PRCT)is more suitable than explanatory randomized controlled trial(ERCT). ERCT is necessary to conduct confirmatory study for new indications of postmarketing Chinese medicine. According to the specific purpose, design type and evaluation index, we choose the suitable sample size estimation method, and determine the corresponding parameters. We must estimate and determine the suitable sample size according to the specific clinical evaluation design, with comprehensive consideration of the test power, research cost and practical operability.