“…Existing studies have focused on modeling learners' learning paths (Chen, Culpepper, et al, 2018;Wang et al, 2018), accelerating learners' memory speed (Reddy et al, 2017), providing model-based sequence recommendation (Chen, Li, et al, 2018;Lan & Baraniuk, 2016;Xu et al, 2016), tracing learners' concept knowledge state transitions over time (Lan et al, 2014), and selecting materials for learners optimally based on model-free algorithms (Li et al, 2018;Tang et al, 2019). However, explicit models are typically needed to characterize learners' learning progresses in these studies.…”