In this study, Dutch gastroenterologists who are certified for performing colonoscopies on FIT-positive patients in the Dutch population screening program are trained in optical diagnosis with validated methods. After this training, an ex- and in-vivo test phase leads to "accreditation" and endoscopists will be observed in their optical diagnosis for 1 year. During this year, half of the endoscopists will be randomized towards 3-monthly feedback and the other half will receive feedback on their results after 1 year. The endoscopic prediction of endoscopists on polyp histology will be compared to histopathological outcome.