When I was in high school I explained a lot of things to my class mates. Later during the first time of my studies, I was hired as a private tutor. Although I was earning some money, I had very mixed feelings about that job. My impression is that only a few students really learned something and most students are only attending since it was their parent's will. Or they needed to have a good grade in the next exam (Which was - to some limit - always possible if there were enough meetings during the weeks before the exam) - But even in that cases, I had the feeling that they only repecipes from me and were drilled to an particular type of questions, ans as soon as a new topic with different type of questions came up they were completly lost. This was maybe also the reason why many students are taking private lessons over a period of severeal years (if not longer).
Is there some reference or some evidence if taking private lessons in mathematics really helps? If yes, in which way or to what limit?