A study that investigates factors affecting the health and illness of populations, and serves as the foundation for interventions (treatment) made in the interest of public health and preventive medicine. 
An epidemiological or population study is considered a cornerstone methodology of public healthresearch for identifying risk factors for disease and determining optimal treatment. However, as such a study is usually based on observational or survey design it can only generate a hypothesis. For testing a hypothesis a randomized controlled trial is essential.