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)