I would like first to express my deep thanks to my God for giving me the ability to complete this thesis. This thesis is the culmination of major combined effort of great number of people, all of whom deserve special commendation. My sincere thanks and appreciation go to my supervisors: Prof. A. Meidan for the effective guidance, valuable advice, useful comments and encouragement throughout the research and the preparation of the thesis and for his continuous support during many frustrating moments. I am also greatly indebted to Dr. J. Padmore who has afforded me constant support and help, her valuable comments in statistical analysis and methodological issues are unforgettable