instance, complements can influence barriers of entry by either lowering or raising it e.g. Apple providing set of tools to develop apps, lowers barriers to entry; make substitution easier e.g. Spotify replacing CDs A strategy consults job is to identify complements and apply them to the forces above. Usage Strategy consultants occasionally use Porter's five forces framework when making a qualitative evaluation of a firm's strategic position. However, for most consultants, the framework is only a starting point and value chain analysis or another type of analysis may be used in conjunction with this model. Like all general frameworks, an analysis that uses it to the exclusion of specifics about a particular situation is considered naïve . According to Porter, the five forces framework should be used at the line-of-business industry level; it is not designed to be used at the industry group or industry sector level. An industry is defined at a lower, more basic level: a market in which similar or closely related products and/or services are sold to buyers (see industry information). A firm that competes in a single industry should develop, at a minimum, one five forces analysis for its industry. Porter makes clear that for diversified companies, the primary issue in corporate strategy is the selection of industries (lines of business) in which the company will compete. The average Fortune Global 1,000 company competes in 52 industries. Criticisms Porter's framework has been challenged by other academics and strategists. For instance, Kevin P. Coyne and Somu Subramaniam claim that three dubious assumptions underlie the five forces: That buyers, competitors, and suppliers are unrelated and do not interact and collude. That the source of value is a structural advantage (creating barriers to entry). That uncertainty is low, allowing participants in a market to plan for and respond to changes in competitive behavior. An important extension to Porter's work came from Adam Brandenburger and Barry Nalebuff of Yale School of Management in the mid-1990s. Using game theory, they added the concept of complementors (also called "the 6th force") to try to explain the reasoning behind strategic alliances. Complementors are known as the impact of related products and services already in the market. The idea that complementors are the sixth force has often been credited to Andrew Grove, former CEO of Intel Corporation. Martyn Richard Jones, while consulting at Groupe Bull, developed an augmented five forces model in Scotland