▁consistent ) ▁and ▁an ▁empir ical ▁point ▁of ▁view ▁( there ' s ▁no ▁guarantee ▁that ▁the ▁parameter , ▁even ▁if ▁it ▁makes ▁concept ual ▁sense , ▁remains ▁stable ▁when ▁a ▁new ▁policy ▁– ▁the ▁government ' s ▁subs id y ▁– ▁is ▁introduced ). ▁Cal ibr ations ▁of ▁models ▁of ▁industry ▁structure ▁on ▁the ▁other ▁hand ▁have ▁generally ▁found ▁that ▁most ▁industry ▁are ▁neither ▁Cour not ▁( in ▁which ▁case ▁the ▁Br ander – S pen cer ▁policy ▁pres cription ▁would ▁be ▁an ▁export ▁subs id y ) ▁nor ▁Ber tr and ▁( in ▁which ▁case ▁the ▁optimal ▁policy ▁is ▁an ▁export ▁tax ). ▁As ▁a ▁result , ▁it ▁is ▁hard ▁to ▁say ▁in ▁which ▁indust ries ▁exactly ▁the ▁Br ander – S pen cer ▁policy ▁pres cription ▁may ▁apply . ▁ ▁Agricult ure ▁Marie ▁Th urs by ▁has ▁used ▁an ▁extended ▁version ▁of ▁the ▁model ▁to ▁examine ▁international ▁trade ▁in ▁whe at , ▁ 6 0 % ▁of ▁which ▁is ▁produced ▁by ▁the ▁United ▁States ▁and ▁Canada . ▁Th urs by ▁includes ▁market ing ▁bo ards , ▁possibility ▁of ▁a ▁mon op son y , ▁and ▁a ▁variety ▁of ▁government ▁policies ▁in ▁the ▁analysis . ▁She ▁finds ▁that ▁while ▁there ▁are ▁significant ▁econom ies ▁of ▁scale ▁in ▁the ▁industry , ▁the ▁bar riers ▁to ▁entry ▁are ▁not ▁high ▁and ▁regardless ▁of ▁the ▁extent ▁of ▁market ▁power ▁that ▁US ▁fir ms ▁have ▁in ▁the ▁industry , ▁the ▁optimal ▁policy ▁is ▁actually ▁an ▁export ▁tax , ▁rather ▁than ▁a ▁subs id y .