There are many free sample size and/or power “calculators” available on the internet, aimed at assisting researchers in planning the number of subjects to include in studies. Not all are of equal quality, and before deciding to rely on one of these free calculators, be sure of what you’re getting.

**What to look for:**

**Statistical method(s) stated clearly:** Using information provided on the sample size calculator website, you should be able to determine what statistical method(s) form the basis for the sample size/power calculations provided. This is essential information. The statistical methods used for estimating the sample size/power must match the researcher’s planned statistical methods for analysing the study data at the time of study completion.

**Reference provided:** Ensure that the website provides a trusted, published reference for the statistical methods used to calculate sample size/power. Peer-reviewed journal articles are usually reliable sources of information, as are statistical encyclopaedias.

**Assumptions:** In addition to knowing what statistical method(s) are being used, it’s important to be aware of assumptions necessary for the methods to be appropriate. For example, is it necessary to assume a normal distribution? equal variances across groups? other conditions?

**Has the calculator been validated:** Does the calculator website describe what steps were taken to ensure that the sample size/power calculator actually does what it’s supposed to do? If not, contact the website owner to get an answer to this question.