PRAttention,Time-Sharing,andWorkloadOVERViEWInChapter3,muchofthefocusofourdiscussionwasonhow peopleattendedtoorperceivedtwo(or more)channelsofperceptualinformationatonce,thatis,dividedattention.Weemphasizedtheconceptofproximitytoaccountformuchofthatsuccess,andalsotheconceptofanoptimalallocationoftheattentionalspotlight,toeffectivelyselectfromamong multiplechannels.Insubsequentchapters,wediscussedfurtherstagesofinformationprocessingrelatedtocognitionandworkingmemory,andresponseselectionandexecution.Nowitisappropriatetoreexaminetheissueofdividedattention,butthistimefromtheperspectiveofdividingattentionbetweentasks,andnotjustbetweenperceptualchannels.Thedifferenceisthatataskmayencompassmultiplestagesofinformationprocessing,andalsowillbedefinedbyaspecificgoal(whichcaninturnbecharacterizedbyaperformancemetric).Thus,goaloFthetask.ofivingistokeepthecarcentetc4utthelaneata.specifiedspee&ithou4.)coffision.Severalpejiiialchannc1&may_buse&toachie-ve-that-goaLarnLthe..stagepoiiise1ectionand=exeGutlonaretrittcal-aswell-thatof.pereeption.Examplesofdividedattentionbetweentasksareplentiful.Our unfortunatevandriverwhowastryingtodiagnosetheproblemofhisautomateddispatcherwhiledrivingisjustonesuchexample.Anotheriswhenyourehearsethephonenumberofthepizzaplacewhilelisteningtoyourcompaniongiveyouapizzatoppingrequest.Stillanotherisrememberingthemessageononecomputerscreenwhileyougothroughtheactionsnecessarytopullupasecondscreen.Also,considerthepointguardonthebasketballteam whodeftlycontrolsherdribblearoundadefenderwhilemonitoringforthecentertocuttothe openspotunderthebasket.Theaboveexampleshaverangedfrommajorfailuresofdividedattention(theunfortunatevan driver)tobrilliantsuccesses,withvaryingdegreesofsuccessinbe-439440Chapter11Attention,lime-SharingandWorkloadtween.Inthefirsthalfofthischapter,wefocusonfourclassesofmechanismsthataccountforvariabilityinthesuccessofdividedattentionortime-sharing.Thenweturntoadiscussionof mentalworkload,oneofthemostimportanthumanfactorsissuesinthestudy ofdividedattention.Inourdiscussionofdividedattention,weofferthebaselineofsingletaskperformance.Twotasksperformedconcurrently(time-shared)aswellaseachwouldbeperformedundersingletaskconditionsdefinesperfecttime-sharingorperfectparallelprocessing.Whenoneortheotherdropsbelowitsbaseline,wesaythereisadual taskdecrementordualtask interference.Ouremphasisisonunderstandingthe fourprocessingmechanisms—resourcedemand,switchingandallocation,structure,andconfusion—thataccountforthefailuresandsuccessesoftime-sharing.Weshouldnotethatthesefourmechanismsarebynomeansmutuallyexclusive,andwehaveseensomeexamplesoftheirdifferentmanifestationsinearlierchaptersofthisbook.However,itisalsotruethateachhasbeenassociatedwithdifferenttheoreticalpositions.Indeed,onepreliminarynote shouldbe made.Theconceptof“attention”hassometimesbeenconsideredtobeindivisibleand,hence,“dividedattention”aninherentcontradiction.Inourwriting,werejectthisposition,butforthosewhomaybeuncomfortable,weofferthealternativephraseof“concurrentprocessing”or“time-sharing”asameansofdescribinganoperatordoingtwothingsat once(independentlyofwhereattentionmaybefocused).MECHANISMSOF TiME-SHARINGAutomaticityandResourcesInChapters6and7,welearnedofthedistinctionbetweenautomaticandcontrolledprocessesinperception(Schneider&Shiffrmn,1977).Automaticprocesseswereformedonthebasisofaconsistentmappingbetweenstimulusandsomefurthercategorization,andtheywererapid,accurate,an&elativelresourcefree.Suchamechanismexplainstheautomaticprocessmgoffamiliarperceptualstimulisuchasletters(LaBerge,1973),consistentlyassignedtargets(Schneider&Fisk,1982;Schneider&Shiffrin,1977),andrepeatedsequencesofstimuli(Bahrick,Noble,&Fitts,1954;Bahrick&Shelly,1958),aswellastheautomaticperformanceofhabitualorhighlypracticedmotoractssuchassigningone’sownnameorshootingajumpshot.Whendescribingtheresonseendofprocessing,thispmaticityofperformçeisreferredasthotai4r12gr(Keele,1973;Summers,1989),asdiscussedinChapter10.Thusitisnotsurprisingthatautomaiçroceigcanbereadilyandefficientlytimesharedwithothermoredemandingtasks(SchneidF&Fisk,i982)—————--—-—--—---——----———--—ThEconceptofautomaticitycanbeapplied toawiderangeofactivities.Walkingisanearlyautomaticactivityformostofus(aslongastheground—thestimulus—thatsupportswalkingisconsistentandpredictable),sowecaneasilyandeffortlesslywalkanddoothertaskswithperfecttime-sharing.Evenifthesurfaceisnotperfectlyfiat,sothatwalkingisalittlemoredifficult(isnolongerentirelyautomatic),wecanstillperfectlytime-share.However,wehavenowbroughttwootherfactorsintotheequationofpredictingmultiple-taskperformance:ffurinddifficiiityChapter11Attention,lime-SharingandWorkload441Thefactorsofeffortanddiflicultysuggestth?taomaticityisnotreallyanallornonephenomenon,butrathercanbethoughtofasacuuiiEwhichcanberelated,inversely,to themjntlrercesdemandedbyatask(Ihneman,1973).TnaseakofataskdemancaseitsperformanceisfflddineinuaLtask.circumstances.Alternatively,wemayspeakofmoreresources(effort)beingallocatedtoatask,inwhichcaseitsperformancewillbelikelytoimprove.AusefulframeworkforthinicingaboutthiscontinuumistheperfQrmanceresourcefunction,orPRF(Norman&Bobrow,1975),anexampleofwhichisshowninFigure11.1.Single-taskperformanceoccurswhenallresourcesareinvestedinthetask(point1oncurveB)andisthebestthatcanbeobtained.Divertingalargeamountofresourcesawayfromthetaskshowninthetopcurvetoaconcurrenttaskwilldepressperformanceaccordingly,asindicatedbypoint2.Asmoreresourcesarethenreinvestedbackintothetask,performancewillimproveuptopoint3,atwhichnofurtherchangeinperformanceispossible.Tothe rightofpoint3,thetaskissaidtobedata-linked(limitçythequalityofdata,notbythejesourcesinveste4)Datalimitsmaoccurat anylevelofperformance.Rememberingatwo-digitnumberisdata-limited,sinceperfectperformancecan beobtainedwithfewresourcesandfurthereffortwillleadtonoimprovement.Datalimitsalsooccurinvigilancewithlow-intensitysignals,orinunderstandingaconversationinalanguagewithwhichyouareonlyfaintlyfamiliar.Inbothcases,nomatterhowhardyoutry(howmucheffortyouinvest),perfectperformanceisimpossible,andbeyondaparticularlevel,performancegainswillnotberealizedwithmoreEFigure11.1Theperformanceresourcefunctionandpractice.TaskBispracticedoreasy;taskAisunpracticedordifficult.3‘B2d’AResources442Chapter11Attention,lime-SharingandWorkloadeffort.Whenperformancedoeschangewithaddedor depletedresources,thetaskissaidtoberesource-limited,theregiontotheleftof