4Machtmes,K.,and Asher,J.W.A meta-analysis of the effectiveness of telecourses in distance education.The American Journal of Distance Education.2000 .14 1 .27-46.
5Moore,M.G.,＆ Thompson,M.M.1990 .The effects of distance learning:A summary of literature.College Park,PA:American Center for the Study of Distance Education.Monograph,2.
