Analysis of covariance and sample size calculation for comparing means in randomized controlled trials.

, We read the valuable manuscript with the title: Effects of sodium selenite and selenium-enriched yeast on cardiometabolic indices of patients with atherosclerosis: A double-blind randomized clinical trial study that published in J Cardiovasc Thorac Res. 1 Which seems to have shortcomings in the following cases: Firstly, about determined sample size the authors mentioned “ The sample size of the study was defined based on the mean ( ± standard deviation [SD]) of GPX with a confidence interval of 95% and power of 80 %, and estimation 10% of dropouts.” The authors used the sample size formula for estimation a single mean which is not applicable for analytical studies such as randomized control trials. The appropriate formula of sample size for comparing two means is as follows