In Chapter 1, we have seen that findings of empirical research can become relevant for legal reasoning at several levels. This chapter will give a short introduction into empirical research methods. The aim of the chapter is to give lawyers a better understanding of the structure of empirical research. It will not enable anybody to conduct his or her own study. However, readers of this chapter should be better able to understand empirical studies that have been conducted by others, and to evaluate the relevancy and the limits of the findings. There are several kinds of empirical research. Some studies try to interpret social practices, while others attempt to describe social phenomena, and still others try to explain causal relationships. In this chapter, we will focus on the latter two strands of research, in particular on the statistical analysis that is a central part of such studies. An example of descriptive research is a comparative study on the development of university dropouts in Organisation for Economic Co-operation and Development (OECD) countries. We need to perform a statistical analysis for such a descriptive study because it is impossible to observe every case that occurs in reality. Instead, such a study is usually based on a representative sample. However, samples are never a perfect representation of the whole population, that is, all possible cases that occur in reality.