Groenewald, W.H.; Gouws, P.A.; Cilliers, F.P.; Witthuhn, R.C.(Journal for New Generation Sciences, Vol 11, Issue 2: Central University of Technology, Free State, Bloemfontein, 2013)
Van Dyk, D.L.; Bezuidenhout, H.(Journal for New Generation Sciences, Vol 11, Issue 2: Central University of Technology, Free State, Bloemfontein, 2013)