@INBOOK {SPENCER:1991a, title = {Collecting meaningful data on labor use and farm size for economic analysis associated with on-farm trials in sub-Saharan African}, author = {Spencer, D. }, year = {1991}, type = {On-farm research in theory and practice, edited by H.J.W. Mutsaers and P. Walker}, pages = {53--59}, publisher = {International Institute of Tropical Agriculture (IITA)}, address = {Ibadan, Nigeria}, }