AS M ▁should ▁only ▁be ▁compared ▁across ▁air lines ▁with ▁care . ▁For ▁instance , ▁all ▁other ▁things ▁being ▁equal , ▁an ▁air line ▁with ▁a ▁longer ▁average ▁stage ▁length ▁will ▁have ▁a ▁lower ▁C AS M , ▁because ▁fixed ▁costs ▁will ▁account ▁for ▁a ▁lower ▁portion ▁of ▁its ▁total ▁costs . ▁For ▁this ▁reason , ▁to ▁be ▁meaning ful , ▁C AS M ▁compar isons ▁across ▁different ▁air lines ▁generally ▁require , ▁at ▁a ▁minimum , ▁that ▁C AS M s ▁for ▁all ▁air lines ▁be ▁adjust ed ▁to ▁a ▁common ▁stage ▁length , ▁or ▁that ▁the ▁C AS M s ▁be ▁graph ed ▁versus ▁the ▁stage ▁length ▁of ▁all ▁the ▁air lines ▁being ▁compared . ▁ ▁C AS M ▁is ▁generally ▁calculated ▁for ▁a ▁particular ▁air line ▁( or ▁portion ▁of ▁an ▁air line — for ▁instance , ▁an ▁air line ' s ▁Pacific ▁operations ▁as ▁opposed ▁to ▁the ▁entire ▁air line ) ▁for ▁a ▁particular ▁period ▁of ▁time — a ▁year , ▁a ▁quarter , ▁a ▁month ▁and ▁so ▁forth . ▁It ▁can ▁also ▁be ▁calculated ▁for ▁a ▁particular ▁route ▁of ▁an ▁air line . ▁However , ▁when ▁it ▁is ▁calculated ▁for ▁anything ▁other ▁than ▁the ▁entire ▁operation ▁of ▁an ▁air line , ▁the ▁relevant ▁costs ▁will ▁in ev it ably ▁involve ▁cost ▁alloc ations ▁that ▁require ▁jud g ments . ▁For ▁instance , ▁the ▁sal ary ▁of ▁the ▁air line ' s ▁senior ▁management ▁needs ▁to ▁be ▁spread ▁across ▁the ▁different ▁parts ▁of ▁the ▁air line ▁and ▁can ▁be