1. Hawassa University School of Mathematical and Statistical Sciences; P. O. Box 05 Hawassa Ethiopia
2. Oslo Centre for Biostatistics and Epidemiology, Department of Biostatistics; University of Oslo; P. O. Box 1122 Blindern Oslo N-0317 Norway
3. Department of Mathematics and Statistics; P. O. Box 68, FI-00014 University of Helsinki; Helsinki Finland