There are several reason, however for using these concentration ratios cautiously. First in industries where imports are significant. concentration ratios may greatly overestimate the relative importance of the largest firm in the industry. For example, since automobile imports represent about 30 percent of U.S. auto sale, the real four-firm concentration ratio in the automobile industry (which includes Honda's U.S. output as the fourth largest U.S. producer) is not 68 percent (as indicated in the table) but 48 percent, (i.e. 68 percent times 0.70) Second, concentration ratios refer to the nation as a whole, even though the relevant market may be local. For example, the four-firm concentration ratio for the cement industry is 39 percent but because of very high transportation costs only two or three firms may actually compete in many local market. Third how broadly or broadly a product is defined is also very important. For example the concentration ratio in the office machines industry as a whole is smaller than in the personal computer segment of the market. Fourth and as postulated by the theory of contestable markets concentration ratio do not give any indication of potential entrants into the market and of degree of actual and potential competition in the industry. In short concentration ratio provide only one dimension of the degree of competition in the market and although useful they must be used with great caution.