Data are sporadically available (with a 5-year delay) to compute industry concentration ratios, which may help identify market power.
Concentration ratios are the percentage of total industry sales, assets, employment, value added, or output accounted for by an industry's biggest four or eight firms, although any number of firms could be used.
Formally, four-firm concentration ratios are defined as
where Si is the market share of the ith firm and S1 > S2 > S3 > S4.
Concentration declines as you move down the table. Changing industry concentration can generally be interpreted from data for changes in concentration ratios and for the number of firms in the industry. For example, market power among firms producing electronic computing equipment appears to have fallen sharply. Not only has its concentration ratio fallen, but the computer industry had five times as many firms in 1982 as in 1967. This undoubtedly reflects technological advances that are still taking place in computers.
MISSING TABLE ONE!!! (macro12-total 27)