Whether in the laboratory or the field, one of the more important challenges for researchers, particularly those who study treatment effectiveness, is designing research that will have sufficient sensitivity to detect those effects it purports to investigate. The sensitivity of a research design is determined by many factors ranging from sample size, validity and reliability of the measures and the type of statistical analysis. However, the end result is a data set upon which the statistical significance can be tested using procedures that have a high level of statistical power. This book examines the concept of design sensitivity and explains statistical power and the elements that determine it. Through explanations of examples, Lipsey discusses a variety of topics, such as the many factors that degrade design sensitivity, effect size parameters and various approaches to assessing it and how to estimate statistical power for various statistical tests.