# Q-Matrix - A $J \times K$ matrix of zeros and ones. $q_{jk}$ indicates whether skill $k$ is required to correctly answer item $j$. - $X_{ij}$ be the response of examinee $i$ to item $j$, $\mathbf\alpha_i = \{\alpha_{ik}\}$ be the binary skill vector. - A cognitive design matrix that explicitly identifies the cognitive specification for each item.