Written by a renowned statistician, this book presents the basic ideas behind the statistical methods commonly used in studies of human subjects. It is an ideal guide for advanced undergraduates who are beginning to do their own research. It presents the basic principles in a non-mathematical way and is accessible to a wide audience with little background in statistics. The text begins at the point when a study is originally conceived and moves in order through all stages to the final report writing, covering both observational and experimental (intervention) studies. Due to the primordial importance of the proper design of a study, much of the material concentrates on this aspect.