[Industry concentration measures calculated with Compustat data, which cover only the public firms in an industry, are poor proxies for actual industry concentration. These measures have correlations of only 13% with the corresponding U.S. Census measures, which are based on all public and private firms in an industry. Also, only when U.S. Census measures are used is there evidence consistent with theoretical predictions that moreconcentrated industries, which should be more oligopolistic, are populated by larger and fewer firms with higher price-cost margins. Further, the significant relations of Compustatbased industry concentration measures with the dependent variables of several important prior studies are not obtained when U.S. Census measures are used. One of the reasons for this occurrence is that Compustat-based measures proxy for industry decline. Overall, our results indicate that product markets research that uses Compustat-based industry concentration measures may lead to incorrect conclusions.]

Industry concentration measures calculated with Compustat data, which cover only the public firms in an industry, are poor proxies for actual industry concentration. These measures have correlations of only 13% with the corresponding U.S. Census measures, which are based on all public and private firms in an industry. Also, only when U.S. Census measures are used is there evidence consistent with theoretical predictions that more-concentrated industries, which should be more oligopolistic, are populated by larger and fewer firms with higher price-cost margins. Further, the significant relations of Compustat-based industry concentration measures with the dependent variables of several important prior studies are not obtained when U.S. Census measures are used. One of the reasons for this occurrence is that Compustat-based measures proxy for industry decline. Overall, our results indicate that product markets research that uses Compustat-based industry concentration measures may lead to incorrect conclusions. The Author 2008. Published by Oxford University Press on behalf of The Society for Financial Studies. All rights reserved. For Permissions, please e-mail: [email protected]., Oxford University Press.