title: Controlled clinical trials: the controls are the key.
authors.forename: D J
authors.initials: DJ
authors.lastname: Ahnen
mesh_terms: D015209:Cholangitis, Sclerosing / Q000150:complications; D003093:Colitis, Ulcerative / Q000150:complications; D015179:Colorectal Neoplasms / Q000209:etiology; D018849:Controlled Clinical Trials as Topic / Q000379:methods*; D006801:Humans; D018579:Patient Selection; D012107:Research Design; D012307:Risk Factors