Relevant literature:

Bloom, B. S. (1984). The 2 sigma problem: The search for methods of group instruction as effective as one-on-one tutoring. Educational Research, 13, 4-16.

Kulik, J. A., & Fletcher, J. D. (2016). Effectiveness of intelligent tutoring systems: A meta-analytic review. Review of Educational Research, 86, 42-78.

Steenbergen-Hu, S., & Cooper, H. (2013). A meta-analysis of the effectiveness of intelligent tutoring systems on K12 students’ mathematical learning. Journal of Educational Psychology, 105, 970-987.

Steenbergen-Hu, S., & Cooper, H. (2013). A meta-analysis of the effectiveness of intelligent tutoring systems on college students’ academic learning. Journal of Educational Psychology, 106, 331-347.

U.S. Department of Education, Office of Educational Technology, Expanding Evidence Approaches for Learning in a Digital World, Washington DC, 2013.

VanLehn, K. (2011). The relative effectiveness of human tutoring, intelligent tutoring systems and other tutoring systems. Educational Psychologist, 46, 197-221.

Woolf, B. P. (2009). Building intelligent interactive tutors: Student-centered strategies for revolutionizing e-learning. Burlington MA: Morgan Kaufman Publishers.