Comparative Effectiveness of Split-Dose Colonoscopy Bowel Preparation Regimens

Inclusion Criteria: * \> 18 years of age, and * being scheduled for outpatient elective screening, surveillance or diagnostic colonoscopies, and * the provider ordering the colonoscopy giving permission to enroll the patient. Exclusion Criteria: * Patients who are \<18 years * undergoing inpatient colonoscopy * those with contra-indications to receiving the standard 4L PEG-ELS colonoscopy bowel preparation (e.g., allergy to PEG) will be excluded * Those with a preference for a specific bowel preparation will be excluded. * The investigators are excluding inpatient colonoscopies because they account for a very small fraction of the total colonoscopies performed. * Also, inpatient colonoscopies are often performed for urgent reasons such that rapid bowel preparation procedures are followed. * In addition, because the objective of inpatient colonoscopy is often not to look for small polyps, the threshold for "adequate" bowel preparation quality might be different from that for outpatient procedures. * In addition, for patients undergoing more than 1 colonoscopy during the study period, only their first colonoscopy will be included in the primary analysis. * Patients who are undergoing a repeat colonoscopy for to a recent inadequate colonoscopy examination with poor bowel preparation will be excluded.