Msuya, C.P.; Duvel, G.H. (Gustav Heinrich)(South African Society for Agricultural Extension, Department of Agricultural Ecomomics, Extension and Rural Development, University of Pretoria, 2007)
Jibba, K.B.; Duvel, G.H. (Gustav Heinrich)(South African Society for Agricultural Extension, Department of Agriculture, Ecomomics, Extensions and Rural Development, University of Pretoria, 2008)
Pienaar, Juanita (Juanita Magrietha), 1965-; Du Plessis, Willemien (Wilhelmina), 1958-; Olivier, N.J.J. (Nicolaas Johannes Jacobus)(The Verloren van Themaat Centre for Public Law Studies, UNISA, 2013)
Stones, W.A.; Terblanche, S.E. (Stephanus Esaias), 1940-(South African Society for Agricultural Extension, Department of Agriculture, Ecomomics, Extensions and Rural Development, University of Pretoria, 2012)
Du Plessis, Willemien (Wilhelmina), 1958-; Pienaar, Juanita (Juanita Magrietha), 1965-; Olivier, N.J.J. (Nicolaas Johannes Jacobus)(The Verloren van Themaat Centre for Public Law Studies, UNISA, 2012)