Porter, M 1985, Competitive advantage-creating and sustaining superior performance, Free Press, New York, as cited in Morschett, D, Swoboda, B & Schramm-Klein, H 2006, 'Competitive strategies in retailing - an investigation of the applicability of Porter's framework for food retailers', Journal of Retailing and Consumer Services, vol. 13, pp. 275-87; Porter, M 1980, Competitive strategy: techniques for analyzing industries and competitors, Free Press, New York, as cited in Morschett, D. Swoboda, B & Schramm-Klein, H 2006, 'Competitive strategies in retailing - an investigation of the applicability of Porter's framework for food retailers', Journal of Retailing and Consumer Services, vol. 13, pp. 275-87.