This article introduces statistical methods for describing and summarizing the results of studies and introduces statistical principles that will guide the psychiatric researcher in the evaluation and interpretation of clinical research. We discuss relatively easy-to-use and informal methods for describing and comparing data. Our aim is to develop methods for investigating relationships among variables, to learn about the effect of one variable upon another. Once we have observed an apparent relationship between variables, an important question to be addressed is whether or not this observed relationship is causal in the sense that a change in one variable causes a changes in the other. We discuss and illustrate principles related to the evaluation of the nature of the association among variables. Throughout the article, principles and methods will be illustrated by examples and case studies based on data sets primarily from the psychiatric research literature.