So far in this book, we have looked at how you can develop a research question, measure the variables involved, and look for relationships among them to provide answers to the research question. A further important factor underlies this whole process, however. To do all these things, you must select a set of observations to look at, and your selection of cases can sharply affect or even distort what you will find. Further, it is often the case that the “selection” occurs by subtle processes other than your own choice. In this chapter, I want to alert you to the importance of case selection (whether it is done by you or by nature) and to show you some basic principles that will help you to select cases in ways that will allow you a clean examination of your research question.