Power analyses(samples size calculation)is critical in protocol design for clinical trials.Adequate power ensures the credibility and reliability of the clinical trial results.In this article,omega-3 fatty acid supplementation study is chosen as an example to explain the power analysis in practice during the study design.First,obtaine the parameter estimates from the high-quality literature review and relevant systematic review(meta analyses)results.Then,calculate the sample size under different parameter settings and select the final patient number according to the clinical practice.Based on the above information,statistical simulation is performed to assume diverse possible combination of the outcome in real clinical trial.Further,under the specific determined sample size,the simulation pointing out the different positive or negative results when the real clinical trial is conducted.The determination of sample size of a clinical trial should be based on both the clinical and statistical considerations.