Four forces -- the bargaining power of customers, the bargaining power of suppliers, the threat of new entrants, and the threat of substitute products -- combine with other variables to influence a fifth force, the level of competition in an industry. Each of these forces has several determinants:
The bargaining power of customers buyer concentration to firm concentration ratio bargaining leverage buyer volume buyer switching costs relative to firm switching costs buyer information availability ability to backward integrate availability of existing substitute products buyer price sensitivity price of total purchase
The bargaining power of suppliers supplier switching costs relative to firm switching costs degree of differentiation of inputs presence of substitute inputs supplier concentration to firm concentration ratio threat of forward integration by suppliers relative to the threat of backward integration by firms cost of inputs relative to selling price of the product importance of volume to supplier
The threat of new entrants the existence of barriers to entry economies of product differences brand equity switching costs capital requirements access to distribution absolute cost advantages learning curve advantages expected retaliation government policies
The threat of substitute products buyer propensity to substitute relative price performance of substitutes buyer switching costs perceived level of product differentiation
References: Porter, M. (1979) "How competitive forces shape strategy", Harvard Business Review, March/April 1979. Porter, M. (1980) Competitive Strategy, The Free Press, New York, 1980.