RELATION OF PROFIT RATE TO INDUSTRY 299 concentration measures are theoretically significant. Since profit-rate data are seldom available for firms operating within a sphere so narrow ensus "product, however, we must we are even T to match profit data with concentration measures, confine ourselves to seeking"significant""Census industries To what extent do Census"industries"correspond to theoretical industries as defined? So far as they tend to represent different groups of outputs serving different needs of users-like firearms or fertilizers-they may tend to correspond, but they may also deviate in important respects from theoretical industries. First, although the Census industry occasionally includes only a single group of close substitute outputs-as in the cigarette industry--it commonly includes several technologically related output groups, identifiable as Census products, within each of which there is evidently close substitution but between which there is or may be slight inter-group substitutability for buyers. " The steel industry, ,which includes armor plate, axles, concrete reinforcing bars, etc. is a case in point several or many theoretical industries are potentially included in this Census industry. Second, the Census industry may be so defined as to exclude entirely close substitutes for the outputs which it includes. When the cane sugar industry is defined so as to exclude beet sugar, this is very obviously the case. Third, the Census industry, because it always includes the entire national supply of the products it contains, frequently may lump together several local or regional industries producing a given commodity, i. e, several output groups which have poor intergroup substitutability at going prices because of transport costs. This is evidently true of bakery roducts or common brick. If a Census industry is not guilty of any of these deviations from the theoretical norm, of course, it will tend o approximate a theoretical industry in that it will include a single group of close substitute outputs and exclude no close substitut for the Remembering these potential discrepancies between theoretical and Census industries, we have analyzed the 340 Census industries in search of the answers to two questions. First, which of them correspond closely to theoretical industries, so that their concentra tion measures obviously qualify as theoretically significant? Second, since there are few of these, for which other Census industries are the received concentration measures theoretically significant in that the Census industry concentration is representative of the true con- centrations for the several theoretical industries which the Census industry may contain? Obviously suspect is the Census industry which includes several