Choi 1993.

Methods	Prospective cohort study
Participants	41 ulcerative colitis patients who developed colorectal cancer 19 patients had been undergoing colonoscopic surveillance and 22 had not Patients with a duration of disease of 8 years or more with extension of the disease proximal to the sigmoid colon were enrolled in the prospective surveillance program All patients with high grade dysplasia, a dysplasia‐associated lesion or mass, or carcinoma were advised to undergo colectomy
Interventions	Colonoscopic surveillance versus no surveillance
Outcomes	1. Duke's stage 2. 5‐year survival rate 3. death
Notes	Attempts to control for bias: 1. statistical analysis: (a) survival distributions were calculated by the product‐limit method of Kaplan and Meier (b) statistical significance of differences between distributions was analysed by the Tarone‐Ware method. (c) differences in age distribution at the time of onset of ulcerative colitis and the diagnosis of carcinoma, and duration of ulcerative colitis before development of carcinoma were analysed using the Mann‐Whitney test 2. Surveillance was defined as colonoscopic biopsy study performed with an intent to screen for neoplasia based on long duration of disease without any concomitant symptoms or signs to suggest neoplasia before the procedure