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1、MARKETDEFINITION:AUSER'SGUIDEFranklinM.FisherJaneBerkowitzCarltonandDennisWilliamCarltonProfessorofEconomics1MassachusettsInstituteofTechnology1.Introduction:thePurposeofMarketDefinitionMarketdefinitionhasbecomeanecessarypartofeveryantitrustcase,andthereisnoavoidingdiscussingit.Asweshallsee,marketde
2、finitioncanbeausefultool,awaytobeginorganizingthematerialthatmustbestudied.Unfortunately,toooftenmarketdefinitionbecomesanendinitself--theprincipal,indeed,sometimesalmosttheonlymattertobeanalyzed.Whenthathappens,marketdefinitionstopsbeingausefulorganizingdeviceandbecomesashibbolethofantitrustanalysi
3、s.Tounderstandwhyitisimportanttoavoidthisandhowtohandlemarketdefinitionrequiresanexaminationnotsomuchofrulesformarketdefinitionasofthepurposesofthemarketdefinitionexercise,itsusesanditslimitations.Thefirstthingtounderstandaboutmarketdefinitionisthathowitisdonedependsonthepurposeforwhichitisused.Thus
4、,wheretheissuetobeinvestigatedisthatofthemarketpowerandbehaviorofaseller,themarketwilloftenbedefineddifferentlythanwherewhatisinquestionisthemarketpowerandbehaviorofabuyer.Tofacilitateexposition,Ishallusuallyassumethatwearetalkingofsellers.Beginbyconsideringanallegationofsingle-firmmonopoly.Whydoweu
5、ndertakethemarketdefinitionexercise?Wedosobecausewewishtomeasurethemarketshareoftheallegedmonopolistanddrawconclusionstherefrom.Similarly,inacasewithanallegationofcollusionorinthecaseofaproposedmerger,weareinterestedinmarketdefinitionsothatwecancalculatesomemeasureofconcentration--thesedays,generall
6、ytheHirschman-HerfindahlIndex(HHI)--anddrawsomeconclusionsfromthatmeasurement.Tobesure,suchmeasurementsarenottheendoftheinvestigation.Easeofentrymustalsobeconsidered,andonemightreasonablysaythatsuchaconsiderationrequiresonetoknowwhatitisthatisbeingentered.Thatisfairenough,and,asweshallsee,properanal
7、ysiswillconsiderallthefactsinvolvedinbothmarketshareorconcentrationand1Iwasawitnessforonesideortheotherinmost(notall)ofthecasesdiscussedinthispaper.Inparticular,IwastheprincipaleconomicwitnessforIBMin
