Although this book is not about setting up and performing medical research, it is important to discuss the different study designs that are used within medical research. This is mainly due to the fact that the use of an effect estimate (and sometimes also the use of a statistical method) depends on the study design. Basically, medical studies can be divided into observational studies and intervention studies. In an observational study the researcher only observes, i.e. measurements are made on the subject without influencing the subject. The different measurements are then related to each other. In an intervention study, the subject is not only observed, but there is also some kind of intervention performed. In fact, the intervention is the most important part of the study, because the aim of the study is to investigate the effect of the intervention.