No author has a financial or proprietary interest in any material or method mentioned.

One of the most frequent concerns of the researches is sample size calculation. There are many equations to calculate sample size one should be selected correctly based on the type of the study and data type. Any mistake can mislead the researcher and so the study results. Even an overestimation can cause waste of cost, time and material. Either statisticians or other researchers frequently use online advice on sample size calculation. Online sample size calculators have been searched, reviewed and checked for the calculated results based on known formulas for common research objectives. Considering the most common types of health study objectives regarding sample size calculation (estimating proportion or mean, comparison of two proportions or two means), from 60 website or software, only 5(8.3%) provided all four types of calculations. Overall exact calculation rate was 8.3% and 16.1% per calculation and per site, respectively. Many of the sites just calculate sample size for estimating proportions and most of the results lack the accuracy in calculation.

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