Testlet design is very popular in educational and psychological assessments. This article proposes a new cognitive diagnosis model, the Multiple-Choice Cognitive Diagnostic Testlet (MC-CDT) model for a test with testlets consisting of multiple-choice (MC) items. The MC-CDT model uses the original examinees' responses to MC items instead of dichotomously scored data (i.e. correct or incorrect) to retain the information from different distractors and thus enhance the MC items' diagnostic power. The MCMC algorithm is adopted...