Levels of Evidence Pyramid created by Andy Puro, September 2014
An observational study is a study in which the investigator cannot control the assignment of treatment to subjects because the participants or conditions are not being directly assigned by the researcher.
In an experimental study, the investigators directly manipulate or assign participants to different interventions or environments.
Formal Trials versus Observational Studies (Ravi Thadhani, Harvard Medical School)
Study Designs (Centre for Evidence Based Medicine, University of Oxford)
Learn about Clinical Studies (ClinicalTrials.gov, National Institutes of Health)
Definitions taken from: Dawson B, Trapp R.G. (2004). Chapter 2. Study Designs in Medical Research. In Dawson B, Trapp R.G. (Eds), Basic & Clinical Biostatistics, 4e Retrieved September 15, 2014 from http://accessmedicine.mhmedical.com/content.aspx?bookid=356&Sectionid=40086281.