k-12 Education.Research supports the literature that students who seek and receive tutoring services outperformed their counterparts.[20] However, private tutoring is not always effective in raising academic achievement; and in some schools students commonly skip classes or sleep through lessons because they are tired after excessive external study. This means that the shadow system can make regular schooling less efficient.[3]Teachers who spend more time focusing on private lessons than regular classes can cause greater inefficiencies in the mainstream school system. Situations in which teachers provide extra private lessons for pupils for whom they are already responsible in the public system can lead to corruption, particularly when teachers deliberately teach less in their regular classes in order to promote the market for private lessons.[22]When private tutoring is provided by well trained tutor however the effects can be dramatic, with pupils improving performance by two standard deviations,[23][24] a result known as Bloom's 2 sigma problem.There can be an existing overlap between different types of tutoring with respect to the setting or location of tutoring, the size of tutor-learner pairings/groups, and the method of tutoring provided, for example, one-on-one peer tutoring can take place through online tutoring. Tutoring is typically private since it is exists independent of the system of public and private education, that is, one can be enrolled in public/private schooling and attend private tutoring services.