As part of a re-examination of the concentration-profitability relationship, Gort and Singamsetti (1976) were apparently the first to explicitly ask whether or not 'the profit rates of firms cluster around industry means.' Assigning firms to 3-digit and 4-digit industries, they found to their surprise that the data failed to support the hypothesis that industries have different characteristic levels of profitability. Furthermore, they noted that the proportion of the total variance explained by industry was low (approximately 11 percent, adjusted), did not increase as they moved from 3-digit to 4-digit industry definitions, and did not increase as the sample was restricted to more specialized firms