Sample size calculations are needed to determine how many subjects should be enrolled in the clinical study. The statistician starts from a list of assumptions such as the required levels of the type-I and type-II errors, the study design, and the size of the treatment effect with some estimate of its variability. It is not just a calculation, but rather an in-depth team discussion about these assumptions. Especially the assumed treatment effect requires team input. Typically the statistician will perform a whole series of calculations, including what-if scenarios to find out how sensitive the calculations are to the assumptions made.
Sample size calculations are not an exact science. Formulae and computerized systems are used for the calculations, but always there are uncertainties and strong assumptions. It is in these circumstances that the statistician will present more than just sets of numbers, but will trigger the necessary discussions to find out how realistic the assumptions actually are. This is crucial. It unfortunately happens that a clinical study turn out inconclusive simply because of unrealistic assumptions.
The Emtex statistician has over 25 years of experience in sample size discussions.