The authors evaluated several ways of measuring item difficulty. First, they used the FSAIF1toF3 dataset, whose publisher supplied difficulty levels for each question, to assess how well Deep-IRT's estimated difficulty matched these labels. Second, as an alternative, difficulty was measured from how many students answered each question correctly versus incorrectly, counting only each student's first attempt and only questions with at least 10 respondents. Third, they used PFA together with each student's full learning trajectory to estimate difficulty, restricting this analysis to a subset of five skills. The authors suggest it would be worthwhile to test whether Deep-IRT's difficulty estimates are more accurate than these traditional approaches.