Sample size and statistical power in reproductive research.

The calculation of sample size should be an integral part of the planning stages of all research projects to avoid wasting time, money, and valuable resources. The necessary information for sample size calculations includes the alpha (type I) error level, the beta (type II) error, delta (or the difference you would like to be able to detect), and, for continuous variables, the variance. We review these terms and outline their importance in the calculation of sample size and statistical power. When necessary, the investigator should seek expertise and advice to perform these calculations.