Important considerations in calculating and reporting of sample size in randomized controlled trials

Background: The calculation of the sample size is one of the most important steps in designing a randomized controlled trial. The purpose of this study is drawing the attention of researchers to the importance of calculating and reporting the sample size in randomized controlled trials.    Methods: We reviewed related literature and guidelines and discussed some important issues in sample size calculation and reporting in randomized controlled trials.    Conclusion: The calculation of the sample size is one of the most important steps in designing a randomized controlled trial. According to the CONSORT (Consolidated Standards of Reporting Trials) guideline and other standard guidelines for designing and reporting of RCTs, sample size calculations should be reported and justified in all published RCTs. Because sample size calculations are prone to bias and because of the high ethical and financial costs related to conducting an RCT, we recommend involving a biostatistician at the designing stage of the study and to ask for statistical advice for sample size calculations.