Most methods for fitting cognitive diagnosis models to educational test data and assigning examinees to proficiency classes require the Q-matrix that associates each item in a test with the cognitive skills (attributes) needed to answer it correctly. In most cases, the Q-matrix is not known but is constructed from the (fallible) judgments of experts in the educational domain. It is widely recognized that a misspecification of the Q-matrix can negatively affect the estimation of the model parameters, which may then result in the misclassification of examinees. This article develops a Q-matrix refinement method based on the nonparametric classification method (Chiu & Douglas, in press), and comparisons of the residual sum of squares computed from the observed and the ideal item responses. The method is evaluated with three simulation studies and an application to real data. Results show that the method can identify and correct misspecified entries in the Q-matrix, thereby improving its accuracy.