1. 1. GEORGE BEAL, and JOE M. BUTLER . 'The Diffusion Process ', Special ReportNo. 18, p.2 . Reprint Cooperative Extension Service, Iowa State University of Science and Technology, Ames, Iowa, November, 1962.
2. 2. SAMUEL. COHEN 'A Study of the Utilization of Audiovisual Equipment and Instructional Materials Under Highly Desirable Conditions '. Unpublished Doctor's thesis , Teachers' College, Columbia University, New York, 1956 , pp.5 -10 .
3. 5. HAROLD H. HAILER 'A Study of the Role of Campus AV Services Center in Providing Audiovisual Materials, for the Faculty of Letters and Sciences at the University of Wisconsin '. Unpublished Doctor's thesis , The University of Wisconsin, Madison, Wisconsin, 1955 , p.89 .