That, and/or mastery learning. I haven’t read the research, but I hypothesize the focus on ‘mastery in each subtopic before moving to the next’ is what drives r

That, and/or mastery learning. I haven’t read the research, but I hypothesize the focus on ‘mastery in each subtopic before moving to the next’ is what drives results, while 1-1 tutoring is what drives the speed of attaining them.