@ARTICLE {ALENE:2006e, title = {The production efficiency of intercropping systems annual and perennial crops in southern Ethiopia: a comparison of distance functions and production frontiers}, author = {Alene, A. and Manyong, V. and Gockowski, J. }, journal = {Agricultural Systems}, year = {2006}, volume = {91}, number = {1-2}, pages = {51--70}, issn = {0308-521X}, }