While the use of market models to determine the sources of economic profits is quite new, at least two empirical studies considering the market structure and market value relation have appeared to date. Interestingly, they conflict. Thomadakis (1977) found a close positive relation between the four-firm concentration ratio and relative excess valuation, measured as the difference between the market value of the firm and the book value of tangible assets, all normalized by sales. As a result, Thomadakis argued that "industry concentration plays a role in the determination of excess profits from currently held assets and those expected from the firm's investment options" (p. 185). In contrast, Lindenberg and Ross (1981) reported no significant relation between concentration and Tobin's Q ratio, measured as the ratio of the market value of the firm to the replacement cost of tangible assets. This result disputes Thomadakis's findings and a purely structuralist interpretation of economic profits. According to Lindenberg and Ross, market power may or may not be evident in highly concentrated markets since "high Q's can occur in concentrated or unconcentrated markets and, conversely, low Q's, indicating no significant market power, can occur in markets that have high degrees of concentration" (p. 28).