Based on our observation, the chance that a prerequisite concept is frequently mentioned in its subsequent videos is larger than that a subsequent concept is talked about in its prerequisite videos.
if video V a is a precursor video of V b , and a is a prerequisite concept of b, then it is likely that f (b, V a ) < f (a, V b ), where f (a, V) denotes the term frequency of a in video V. We thus define another feature, namely distributional asymmetry distance (Dad), to calculate the extent that a given concept pair satisfies this distributional asymmetry pattern.