The authors used FSAIF1toF3 in order to evaluate estimated difficulty. They used this dataset as the publisher provided difficulty levels for each question. Another way to measure the difficulty of the question is to take into account how many students answered it correctly and how many didn't. The authors used only first attempt of a student to learn difficulty level and additionally they only considered question for which at least 10 students answered. The authors used PFA and entire student's learning trajectory to estimate difficulty level for the question. They only used questions that belonged to 5 subset of skills. As it was suggested it would interesting to find out if Deep IRT models give more accurate results than the traditional models.