Assessment of endoscopist competence is an increasingly important component of colonoscopy quality assurance. In this study from the Joint Advisory Group on Gastrointestinal Endoscopy, validity evidence is provided for the use of the Direct Observation of Procedural Skills assessment tool in the formative setting during training. In this national UK dataset, overall colonoscopy competence was typically achieved after 200-249 procedures, although certain complex procedural skills ("proactive problem solving" and "loop management") had not reached the threshold for competence even after 300 procedures. These data will help inform the development and/or refinement of certification policies and practices in jurisdictions around the world.