Roohollah, T., R. T. Hamid, and S. Mahdi. “Index of Difficulty Which Is an Effective Factor on Especial Skill Formation in Basketball Free Throw: Index of Learning Difficulty Hypothesis”. Physical Education of Students, Vol. 22, no. 2, Apr. 2018, pp. 83-90, doi:10.15561/20755279.2018.0205.