▁the ▁market ▁share ▁of ▁the ▁four ▁and ▁the ▁eight ▁largest ▁fir ms . ▁Con cent r ation ▁r ati os ▁are ▁usually ▁used ▁to ▁show ▁the ▁extent ▁of ▁market ▁control ▁of ▁the ▁largest ▁fir ms ▁in ▁the ▁industry ▁and ▁to ▁illustrate ▁the ▁degree ▁to ▁which ▁an ▁industry ▁is ▁o lig opol istic . ▁ ▁The ▁standard ▁tools ▁of ▁competition ▁econom ists ▁and ▁competition ▁authorities ▁to ▁measure ▁market ▁concentration ▁are ▁the ▁Her find ahl - H ir sch man ▁index ▁( H HI ) ▁and ▁the ▁concentration ▁r ati os ▁( CR ( n )). ▁These ▁two ▁are ▁known ▁as ▁the ▁traditional ▁struct ural ▁measures ▁of ▁market ▁concentration ▁( based ▁on ▁market ▁shares ). ▁The ▁concentration ▁of ▁fir ms ▁in ▁an ▁industry ▁is ▁of ▁interest ▁to ▁econom ists , ▁business ▁strateg ists ▁and ▁government ▁ag encies . ▁ ▁Two ▁common ▁r ati os ▁▁ ▁The ▁Four - F irm ▁Con cent r ation ▁Rat io ▁measures ▁the ▁total ▁market ▁share ▁of ▁the ▁four ▁largest ▁fir ms ▁in ▁an ▁industry . ▁ ▁The ▁E ight - F irm ▁Con cent r ation ▁Rat io ▁measures ▁the ▁total ▁market ▁share ▁of ▁the ▁eight ▁largest ▁fir ms ▁in ▁an ▁industry . ▁ ▁Usually , ▁these ▁two ▁common ▁r ati os ▁are ▁compar able ▁from ▁industry ▁to ▁industry , ▁while ▁concentration ▁r ati os ▁for ▁other ▁numbers ▁of ▁fir ms ▁can ▁be ▁also ▁calculated . ▁ ▁Me asure ment ▁ ▁N - f irm ▁concentration ▁ratio ▁is ▁a ▁common ▁measure ▁of ▁market ▁structure ▁and ▁shows ▁the ▁combined ▁market