The power of a study is its ability to detect a difference of a given size, given that the difference actually exists. For a fixed sample size, larger differences can be detected with greater power. Excel sheets are given for the calculation of sample sizes to detect specified differences with chosen power and significance; these are known as power calculations.
For each sample size calculation, the size of difference, d, to be detected should be given. 'd' is the minimum difference deemed to be of clinical importance.
In each excel sheet (except time to event) there is a section that gives the required sample size for a specified power, significance and difference. In some sheets there is also a lower section that gives the power of a study to detect a specified difference at a chosen significance level with a set sample size. The latter sections may be useful where numbers are restricted and the power available is useful to know.