Abstract: AIM: Introduction of analysis of covariance (ANCOVA) and the application in clinical trials. METHODS: The method of ANCOVA was illustrated by a clinical case study. The implementation of ANCOVA by EXCEL software was also demonstrated and discussed. RESULTS: Once ANCOVA was employed, statistical significance was observed in the treatment effect on silicosis between new drug group and old drug group, whereas, the effects had no statistical significance when utilizing ordinary analysis. CONCLUSION: In clinical trials, the imbalance property of baseline among test groups may often affect the accurate evaluation to trial result.ANCOVA can be employed in this situation to adjust the baseline imbalance.

Key words: Covariance analysis, Clinical trail