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GroupLens:AnOpenArchitectureforCollaborativeFilteringofNetnewsPaulResnick*,NeophytosIacovou**,MiteshSuchak*,PeterBergstrom**,JohnRiedl***MITCenterforCoordinationScienceRoomE53-32550MemorialDriveCambridge,MA02139617-253-8694Email:presnick@mit.edu**UniversityofMinnesotaDepartmentofComputerScienceMinneapolis,Minnesota55455(612)624-7372Email:riedl@cs.umn.eduFromProceedingsofACM1994ConferenceonComputerSupportedCooperativeWork,ChapelHill,NC:Pages175-186Copyright©1994,AssociationforComputingMachineryABSTRACTCollaborativefiltershelppeoplemakechoicesbasedontheopinionsofotherpeople.GroupLensisasystemforcollaborativefilteringofnetnews,tohelppeoplefindarticlestheywilllikeinthehugestreamofavailablearticles.Newsreaderclientsdisplaypredictedscoresandmakeiteasyforuserstoratearticlesaftertheyreadthem.Ratingservers,calledBetterBitBureaus,gatheranddisseminatetheratings.Theratingserverspredictscoresbasedontheheuristicthatpeoplewhoagreedinthepastwillprobablyagreeagain.Userscanprotecttheirprivacybyenteringratingsunderpseudonyms,withoutreducingtheeffectivenessofthescoreprediction.Theentirearchitectureisopen:alternativesoftwarefornewsclientsandBetterBitBureauscanbedevelopedindependentlyandcaninteroperatewiththecomponentswehavedeveloped.KEYWORDS:Collaborativefiltering,informationfiltering,electronicbulletinboards,socialfiltering,Usenet,netnews,usermodel,selectivedisseminationofinformation.INTRODUCTIONComputernetworksallowtheformationofinterestgroupsthatcrossgeographicalbarriers.Bulletinboardshavebeenanimportantmechanismforthat.Ratherthanaddressinganarticledirectlytoaknownsetofpeople,thewriterpostsitinanewsgroup,apublicplaceavailabletoanyoneinterestedinthetopic.TheUsenetnetnewssystemcreatestheillusionofasinglebulletinboardavailableanywhereintheworld.Itpropagatesarticlessothat,withsomedelays,anarticlepostedfromanywhereintheworldisavailabletoeveryoneelse.Permissiontocopywithoutfeeallorpartofthismaterialisgrantedprovidedthatthecopiesarenotmadeordistributedforcommercialadvantage,theACMcopyrightnoticeandthetitleofthepublicationanditsdateappear,andnoticeisgiventhatcopyingisbypermissionoftheAssociationforComputingMachinery.Tocopyotherwise,ortorepublish,requiresafeeand/orspecificpermission.Recentcountsindicatethattherearemorethan8000newsgroups,withanaveragetrafficofmorethan100MBperday[1].Thenewsgroupscarryannouncements,questions,anddiscussions.Inadiscussion,oftencalledathread,onearticleinducesrepliesfromseveralothers,eachofwhichmayalsoinducereplies.TheJanuary24,1994estimatesofnetnewsparticipationindicatethatmorethan140,000peoplepostedarticlesintheprevioustwoweeks.Therearemanymorelurkerswhoreadbutdonotpostarticles.Clearly,alotofpeoplearegettingvaluefromthesebulletinboards.Infact,netnews’rapidbroadcastnatureandwidespreadreadershiphasreshapedthewaythecomputingcommunityworks.Systemadministratorsdependonnetnewstokeepintouchwiththelatestdevelopmentwork,thelatestsecurityholes,andthelatestbugfixes.Researchersdependonnetnewsasawayofkeepingup-to-dateonnewresearchdirectionsandimportantresultsinbetweenconferences.Manyothersusenetnewsjusttokeepintouchwithotherpeoplearoundtheworld,tolearnaboutnewbooks,newrecipes,newmusic,andwhatlifeinothercitiesislike.Overtheyearsnetnewshasbecomeaprincipalmediumforsharingamongcomputerusers.Evenso,theexperienceofusingnetnewsisnotcompletelysatisfying.Almosteveryonecomplainsthatthesignaltonoiseratioistoolow.Writerscannoteasilytellwhethertheircommentsarevalued,exceptbythevocalfewwhopostresponses.Someseemnottocareaboutreaderinterest,onlyabouttheirownrighttowrite.Moreover,tastesdiffer,sothatnoonearticlewillappealtoallthereadersofanewsgroup.Eachreaderendsupsiftingthroughmanynewsarticlestofindafewvaluableones.Often,readersfindtheprocesstoofrustratingandstopreadingnetnewsaltogether.Netnewsprovidestwomechanismsthathelpreaderslimittheirattentiontoarticleslikelytointerestthem.First,thedivisionofthebulletinboardintonewsgroupsallowsreaderstofocusonafewtopics.Whenthenumberofpostingsinanewsgroupgetstoolarge,itisoftensplitintotwoormorenewsgroupswithidentifiablesubtopics.Second,somenewsgroupsaremoderated.Attemptedpostingstothesenewsgroupsareautomaticallyforwardedtothemoderator,whodecideswhetherornottheybelonginthenewsgroup.Usenetpropagatesonlythosearticlesthatreceivethemoderator’sstampofapproval.Inaddition,softwarepackagesforreadingnetnews(hereafterreferredtoasnewsclients)provideothermechanismsthateasereaders’burdens.First,mostnewsclientsdisplayasummaryoftheauthorandsubjectlineforeachmessageinanewsgroup.Theuserthenindicateswhicharticlesshewouldliketoread.Second,mostnewsclientsdisplayallofthearticlesinaparticulardiscussionthreadtogether.Someinitiallyshowonlythefirstarticleineachthread,allowinguserstoquicklyperusethecurrentdiscussiontopics.Third,somenewsclientsprovidekillfiles.Akillfileidentifiestextstringsthatarenotinterestingtoaparticularuser.Ifauserputsthesubjectlineofanarticleintothekillfile,nofurtherarticlesonthatsubjectwillbedisplayed.Ifauserputstheauthor’snameintoakillfile,nofurtherarticlesfromthatauthorwillbedisplayed.Finally,somene