depth of learning

This is the duration, in days, that learner has demonstrated they can independently recall and accurately answer a question from the layer.

Occasionally, the depth of learning is less than the number of days between the two most recent practices, because the app feels that this duration is not a reliable indicator of the depth of learning

e.g. the learner has flip-flopped between being able to answer independently and accurately and then not, or because the layer was answered very late after a holiday, and the app wants to make sure that the increase is reliable.