A COMMON observation in industrial organization literature is that the measure of 'concentration' used to describe an industry or to relate its structure to performance is an issue of at most secondary importance. Since concentration ratios and other statistics of firm size distribution are highly correlated, it is argued, empirical investigations will show similar results regardless of the choice of index. This paper will demonstrate both theoretic- ally and empirically why that conclusion is unfounded in the case where it would seem most likely to be valid, namely, in a comparison of different concentration ratios. A fortiori, this finding applies to the choice between any alternative measures of firm size distribution, and indeed, to the general problem of selecting empirical variables in regression models. In addition, we shall suggest some economic implications of the statistical results pro- duced by concentration ratios consisting of different numbers of firms.