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 repersent about 30 percent of U.S. auto sale, the real four-firm concentration ratio in the automobile industry (which includes Honda's output as the as the fourth largest U.S. producer) is not percent (as indicated in the table) but 48 percent, 68 percent times 0.70) Second, concentration ratios refer to the nation as a whole, even though the relevant market may be local.