The phd thesis was submitted to the University of Mauritius in April 2000 and Aman Maulloo received his phd degree in July 2000. It is important to note that although Dr Maulloo was registered at the University of Mauritius under the Staff Development programme of the University, the research work was conducted at the Statistical Laboratory, Department of Pure Mathematics and Mathematical Statistics, under the supervision of Professor F.P.Kelly, FRS, and the sponsorship of the World Bank. Aman Kumar Maulloo Copyright 2000