Agreeing on how to best establish what a learner understands isn't simple -- if for no other reason than understanding isn't simple.
Gamification is one response. By embedding diverse achievements into activities and assessments, learning progress can be refracted infinitely. These systems would be able to more flexibly respond to unique learner pathways and abilities, and would further serve as encouragement mechanics -- instead of one carrot stick, there are hundreds. And not just carrots, but every fruit and vegetable imaginable.