▁share ▁of ▁the ▁N ▁largest ▁fir ms ▁in ▁the ▁market . ▁For ▁example , ▁the ▁ 5 - f irm ▁concentration ▁ratio ▁in ▁the ▁UK ▁p estic ide ▁industry ▁is ▁ 0 . 7 5 , ▁which ▁indicates ▁that ▁the ▁combined ▁market ▁share ▁of ▁the ▁five ▁largest ▁p estic ide ▁sell ers ▁in ▁the ▁UK ▁is ▁about ▁ 7 5 %. ▁ ▁N - f irm ▁concentration ▁ratio ▁does ▁not ▁reflect ▁changes ▁in ▁the ▁size ▁of ▁the ▁largest ▁fir ms . ▁ ▁The ▁Her find ahl ▁index ▁avoid s ▁this ▁problem . ▁ ▁Con cent r ation ▁levels ▁ ▁Con cent r ation ▁r ati os ▁range ▁from ▁ 0 ▁to ▁ 1 0 0 ▁percent . ▁The ▁levels ▁reach ▁from ▁no , ▁low ▁or ▁medium ▁to ▁high ▁to ▁" total " ▁concentration . ▁ ▁Per fect ▁competition ▁If ▁there ▁are ▁N ▁fir ms ▁in ▁an ▁industry ▁and ▁we ▁are ▁looking ▁at ▁the ▁top ▁n ▁of ▁them , ▁equal ▁market ▁share ▁for ▁all ▁of ▁them ▁means ▁that ▁CR n ▁= ▁n / N . ▁All ▁other ▁possible ▁values ▁will ▁be ▁greater ▁than ▁this . ▁No ▁concentration ▁If ▁CR n ▁is ▁close ▁to ▁ 0 %, ▁( which ▁is ▁only ▁possible ▁for ▁quite ▁a ▁large ▁number ▁of ▁fir ms ▁in ▁the ▁industry ▁N ) ▁this ▁means ▁perfect ▁competition ▁or ▁at ▁the ▁very ▁least ▁mon opol istic ▁competition . ▁If ▁for ▁example ▁CR 4 = 0 ▁%, ▁the ▁four ▁largest ▁firm ▁in ▁the ▁industry ▁would ▁not ▁have ▁any ▁significant ▁market ▁share . ▁Low ▁concentration ▁