[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.]

Bayesian theory predicts an increase in market participants' reliance on reported current earnings to revise their expectations of future earnings when the uncertainty in future earnings is higher. Prior studies focus on price reactions and find negative associations between measures of earnings uncertainty and investors' reliance on reported current earnings. This study examines analysts' forecast revisions (of future earnings) around the announcements of current period earnings and finds positive associations between measures of earnings uncertainty and analysts' reliance on reported current earnings. The findings suggest that uncertainty measures and discount rates are correlated, and cross-sectional differences in the discount rate taint the interpretation of price reactions as evidence of expectation revisions under uncertainty. This study sheds additional light on the complex relationships among earnings uncertainty measures, price reactions to earnings surprise, and cost of capital.

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.