Power analysis for cross-sectional and longitudinal study designs

Power and sample size estimation constitutes an impor-tant component of designing and planning modern scientific studies. It provides information for assessing the feasibility of a study to detect treatment effects and for estimating the resources needed to conduct the project. This tutorial discusses the basic concepts of power analysis and the major differences between hypothesis testing and power analyses. We also discuss the advantages of longitudinal studies compared to cross-sectional studies and the statistical issues involved when designing such studies. These points are illustrated with a series of examples.