References
- Porter , Michael E. 1980 . Competitive Strategy: Techniques for Analyzing Industries and Competitors. 34 – 46 . New York , New York : The Free Press. .
- Porter , Michael E. 1985 . Competitive Advantage: Creating and Sustaining Superior Performance. 16 New York , New York : The Free Press. .
- Porter , Michael E. 1985 . Competitive Advantage: Creating and Sustaining Superior Performance. 36 New York , New York : The Free Press. .