Reply to: Analysis of covariance and sample size calculation for comparing means in randomized controlled trials
Naimeh Mesri Alamdari, Samad Ghaffari, Neda Roshanravan
Abstract
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TopicsSelenium in Biological Systems · Pregnancy and Medication Impact
The Authors Reply
We profoundly thank Alimohamadi et al^1^ for their great attention and for carefully evaluating the clinical trial “Effects of sodium selenite and selenium-enriched yeast on cardiometabolic indices of patients with atherosclerosis: A double-blind randomized clinical trial study”.^2^ Alimohamadi et al^1^ have proposed some concerns that are better to be discussed here. The answers to the questions are in the order that have been raised.
The first concern is about the determination of sample size. It should be noted that the calculation of sample size is based on the mean change ( ± standard deviation [SD]) of GPX in Hu et al^3^ study. We consider the second group’s mean ( ± standard deviation [SD]) with 20% changes relative to the other group and they were put in the following formula:
Thus, the calculation of sample size is per group and there are between-group comparisons. So, implicitly, we multiplied the calculated sample size by 3 which is much bigger than [sqrt (3)-1]. Unfortunately, it was not clearly explained in the paper. However, the method of determination of sample size was properly done as mentioned by Alimohamadi et al^1^.
Regarding the second concern about the statistical analysis, it should be indicated that we have analyzed data based on mean differences reports. Since the distribution of data was not normal and we were not allowed to use the analysis of covariance (ANCOVA) test, we calculated the difference and used the Kruskal-Wallis Test and compare change between groups. Noted that none of transformations (log, square, inverse,...) were effective to transform data into normal. It is important to emphasize that there is no fault in content and all the analysis methods are properly done. However, it is not completely well-reported in some parts which may confuse the readers.
Competing Interests
None declared by the authors.
Funding
None.
The reference list from the paper itself. Each links out to its DOI / PubMed record.
- 1Alimohamadi Y Sepandi M Analysis of covariance and sample size calculation for comparing means in randomized controlled trials J Cardiovasc Thorac Res 202315213110.34172/jcvtr.2023.3049737654817 PMC 10466466 · doi ↗ · pubmed ↗
- 2Khabbaz Koche Ghazi M Ghaffari S Naemi M Salehi R Taban Sadeghi M Barati M Effects of sodium selenite and selenium-enriched yeast on cardiometabolic indices of patients with atherosclerosis: a double-blind randomized clinical trial study J Cardiovasc Thorac Res 2021134314910.34172/jcvtr.2021.5135047136 PMC 8749358 · doi ↗ · pubmed ↗
- 3Hu Y Mc Intosh GH Le Leu RK Upton JM Woodman RJ Young GP The influence of selenium-enriched milk proteins and selenium yeast on plasma selenium levels and rectal selenoprotein gene expression in human subjects Br J Nutr 201110645728210.1017/s 000711451100042021450115 · doi ↗ · pubmed ↗