Basic Design Considerations

This chapter reviews the reasons why sample size considerations are important when planning a clinical study of any type. The basic elements underlying this process include the null and alternative study hypotheses, effect size, statistical significance level and power, each of which are described. The chapter introduces the notation to distinguish the population parameters that is being estimated with the study, from their anticipated value at the planning stages and also from their estimated value once the study has been completed. It describes the basic properties of the standardised Normal distribution. Methods for calculating confidence intervals for a population mean are indicated together with (suitably modified) how they can be used for a proportion or a rate in larger studies. Finally, a note concerning numerical accuracy of the calculations in the illustrative examples is included.