A clinical trial is a planned experiment designed to evaluate the benefits of a treatment for a specific medical condition. A well- designed clinical trial is the most rigorous research design for evaluating treatments. However, there several situations where it is neither possible nor ethical to conduct a clinical trial. In these situations, observational studies may be more appropriate in evaluating possible associations between a factor of interest (not necessarily treatment) and a particular disease or outcome. In observational study designs, the investigator has no control over the exposure of individuals. Study participants are observed and data are collected but nothing is done to influence either the exposure or the course of events. We will consider three main observational study designs in this article.
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