References :

I4free.con.nz. Review of ESL Fitness Series-Unit 2. http://yippee.i4free.co.nz/html/win/education/title8228.htm

Merit Software. Research Basis for Merit Software.http://www.meritsoftware.com/benef/Research_Basis_for_Merit_Software.pdf.

Skinner, B. F. (1954). The science of learning and the art of teaching. Harvard Educational Review, 24, 86-97.