Clinical studies are used to make generalizations about a population of interest. Bias can be defined as the systematic error in study design or implementation, leading to inaccurate generalizations about this population. There is a potential for bias in all of the clinical studies on sodium phosphate colonoscopy preparation, and this bias may lead to the differing conclusions regarding safety drawn by the authors. A review of some of the relevant literature is presented, as well as a discussion of propensity score analysis, a technique used to help clarify the causal pathway in nonrandomized studies. Based on the available information, it is reasonable to follow the recommendations contained in the consensus document of the American Society of Colon and Rectal Surgeons, American Society for Gastrointestinal Endoscopy, and Society of American Gastrointestinal and Endoscopic Surgeons regarding sodium phosphate colonoscopy preparation.