Karlen 1998a.

Methods	Case control study
Participants	142 patients with ulcerative colitis of at least 5 years duration 40 patients who died from colorectal cancer were compared to 102 control patients matched for age, sex, extent and duration of ulcerative colitis
Interventions	None
Outcomes	Exposure to at least one surveillance colonoscopy
Notes	Attempts to control for bias: 1. Control group was matched for age, sex, extent and duration of disease. Controls had to be alive at the time of death of the patient and to have some part of their colon intact five years prior to the diagnosis of the cancer of the patient 2. Statistical analyses: the relationship between colonoscopic surveillance and colorectal cancer mortality was analysed by the relative risk obtained by the odds ratio Matched analyses were performed using conditional logistic regression analyses The estimated standard deviations of the regression coefficient estimates were used to calculate 95% confidence limits. 3. Only colonoscopies performed with the intention of cancer surveillance were included, index colonoscopies and those being made due to clinical symptoms or signs were excluded Confounders not controlled for: 1. drug treatment (e.g. SASP treatment has been shown to reduce the risk of colorectal cancer)