Chul Ahn & Moonseoung Heo 
Sample Size Calculations for Clustered and Longitudinal Outcomes in Clinical Research [PDF ebook] 

Accurate sample size calculation ensures that clinical studies have adequate power to detect clinically meaningful effects. This results in the efficient use of resources and avoids exposing a disproportionate number of patients to experimental treatments caused by an overpowered study. Sample Size Calculations for Clustered and Longitudinal Outcom