5Davies, R. , & Elias,P. (2003). Dropping out : A study of early leavers from higher education [ R ]. London:Department for Education and Skills.
6Kember, D. ,& Harper, G. (1987) . Implications for instruction arising from the relationship between approaches to studying and academic outcomes[ J]. Instructional Science, (16) :35-46.
7Kennedy, D. , & Powell, P. ( 1976 ). Student progress and withdrawal in the Open University[ J]. Teaching at a Distance, (7) :61-75.
8Malley, J. I. ,Brown,A. P. ,& Williams, J. W. (1976). Dropouts from external studies: A case study of the investigation process [ J]. Epistolodidaktika, (2) : 170-179.
9Simpson,O. (2008). Cost-benefit of student retention policies and practies, economics of distance and online learning: Theory, practice, and research[ M]. New York& London:62-178.
10Smith, P.J. (1979). Performance and attrition in the wake of expanded admission[ R ]. A Comparison of External Students with Parttime and Full-time On-campus Students[ C]. Paper presented at the 5th Biennial General Meeting of the Australian and South Pacific External Studies Association, Perth.
