1. Study design: endoscopist-blinded, prospective, randomized controlled trial
2. Subjects
\- A single-center, randomized, observer-blinded study was performed at Korea University Hospital in Anam. Outpatients were prospectively enrolled. Each group of the patients will receive PEG-Asc or PEG-Asc with simethicone before colonoscopy.
3. Sampling design: Consecutive recruitment of consenting patients
4. Variables Predictor
1. group 1 (PEG-Asc) received 1 L solution of PEG-Asc at 7 p.m the evening before colonoscopy and another 1 L solution of PEG-Asc at 5 hours before procedure;
2. group 2 (PEG-Asc with simethicone) received 1 L solution of PEG-Asc at 7 p.m the evening before colonoscopy and another 1 L solution of PEG-Asc at 5 hours before procedure;Two packs (200 mg/10 mL each) of simethicone (400 mg) was mixed with last 500 mL of additional clear fluid.
5. Primary Outcome: Quality of bowel preparation \[Boston Bowel Preparation Scale, bubble score\]
6. Secondary Outcome: Patients and endoscopists' compliance\[Tolerability, palatability of patient, fatigue score of endoscopist\]
