Study designs in thoracic surgery research.

In this short review, we'll try to specify the differences between evaluation procedures of groups of data, as they present to researchers. The way and time data are gathered defines the type of study is going to shape. When we observe a cluster of data without deliberately interfering with the process we mean to evaluate, we perform an observational study. Observational studies are the main topic of this issue. Upon the contrary, experimental studies imply the direct action of the observer on the study population in order to define the role of a given exposure. The topic of experimental study design will be covered in another issue of this series.