3Braskamp, A. , Brandenburg, Dale C. & Ory, John C. (1984). Evaluating teaching effectiveness: A practical guide[ M]. Bev- erly Hills, CA: Sage. :23-24.
4Gunawardena, C. N. (1997). Analysis of global online debate and the development of an interaction analysis model for examining social construction of knowledge in computer conferencing [ J ]. Educational Computing Research, 17 (4) : 397 -431.
5Kreijnsetal, K. (2011 ). Measuring perceived social presence in- distributed learning groups[ J]. Education Information Technology( 16 ) : 365-381.
6Moore, M. G. (1989). Three types of interaction [ J J. The A- merican Journal of Distance Education, ( 2 ) : 1-6.
7Muirhead, B. .(2000), Interactivity in a Graduate Distance Education School [ J ]. Educational Technology & Society, 3 ( 1 ) : 93 -96.
8Salmon, G. .(2003), The key to teaching and learning online [ M] . London and New York Second Edition. 3-9.
