Inadequate bowel preparation results in decreased rates of cecal intubation, increased rates of missing important lesions, increased patient discomfort, higher risk of complications, prolonged procedure time and increased health-care cost. Recent study reported that Telephone-based re-education (TRE) on the day before colonoscopy significantly improved the quality of bowel preparation and polyp detection rate. However, there is no study to compare the effect of telephone with short message system (SMS) based re-education on the quality of bowel preparation in health screened population. Our goal is to improve the quality of bowel preparation with telephone or SMS based re-education for outpatients undergoing screening colonoscopies. The investigators hypothesise that efforts to improve education and maximise patient compliance during the preparatory period will enhance the efficacy of bowel preparation.