Techniques disclosed herein can determine sequences for educational units by evaluating entropies of multiple potential sequences and biasing selection of next units towards those associated with high sequence entropies. Further, an analysis can determine which units are under-represented (relative to a target proportion) in a past sequence and bias towards inclusion of an under-represented unit in the sequence. The available units that are considered for potential selection can include those matched to a learner's skill (e.g., such that all pre-requisite units have been mastered but the unit itself has not been mastered). Thus, techniques can generate sequences of units that promote unit variation and nonetheless conform to relative unit frequencies and skill level appropriateness.