Additive Factors Model (AFM) and Performance Factors Analysis (PFA) are two popular models of student learning that employ logistic regression to estimate parameters and predict performance. This is in contrast to Bayesian Knowledge Tracing (BKT) which uses a Hidden Markov Model formalism. While all three models tend to make similar predictions, they differ in their parameterization of student learning. One key difference is that BKT has parameters for the slipping rates of learned skills,

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... as the logistic models do not. Thus, the logistic models assume that as students get more practice their probability of correctly answering monotonically converges to 100%, whereas BKT allows monotonic convergence to lower probabilities. In this paper, we present a novel modification of logistic regression that allows it to account for situations resulting in false negative student actions (e.g., slipping on known skills). We apply this new regression approach to create two new methods AFM+Slip and PFA+Slip and compare the performance of these new models to traditional AFM, PFA, and BKT. We find that across five datasets the new slipping models have the highest accuracy on 10-fold cross validation. We also find evidence that the slip parameters better enable the logistic models to fit steep learning rates, rather than better fitting the tail of learning curves as we expected. Lastly, we explore the use of high slip values as an indicator of skills that might benefit from skill label refinement. We find that after refining the skill model for one dataset using this approach the traditional model fit improved to be on par with the slip model.