1. MOA, 1990. Agricultural Statistics: Donkeys, Farms, Animals and Average Size by District and Region. Ministry of Agriculture, Central Statistics Office, Gaborone, Botswana, pp. 78–79.
2. Steel, R.G., Torrie, J.H., 1980. Principles and Procedures of Statistics. A Biometrical Approach, 2nd edn., McGraw-Hill Book, New York.