Concentration Ratios
Concentration Ratios measure how much of the total output of an industry is produced by the n largest firms in that industry, i.e.
where Xi is the fraction of total market sales accounted for by each firm
4-firm Conc. Ratio, CR4=X1+X2+X3+X4
CR8=X1+X2+X3+X4+X5+X6+X7+X8