广告调查（第二版）英文 指导手册 Davis-Instructors-Manual-ch04 Sampling

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AdvertisingResearch:Instructor’sManual

Copyright©2012PearsonEducation,Inc.publishingasPrenticeHall

AdvertisingResearch:Instructor’sManual

Copyright©2012PearsonEducation,Inc.publishingasPrenticeHall

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4. Sampling

ChapterGoals

Afterreadingthischapter,studentsshouldhaveabetterunderstandingofhowto:

• makewell-consideredandappropriatesamplingdecisions.

• describetheroleofsampledefinitionandsampleframesinthesamplingprocess.

• explainthedifferencebetweenprobabilityandnonprobabilitysampling.

• identifythestrengthsandweaknessesofdifferentformsofprobabilityandnonprobabilitysamplingtodeterminesamplesize.

NotestotheInstructor

TheChapterLectureprovidesaguidetokeytopicsandcontent.ThePowerPointslidesarenamed:davis_adresearch_ch4.ppt.Slide4-38containsalinktoanonlinesamplesizecalculator.

ThesupplementalreadingsforthischapterprovidefivecasestudiesdrawnfromthePewCentersthatillustratehowsamplingplansfordifferentresearchstudiesarecustomizedtorespondtoaresearchproject’suniqueinformationalneeds.

ChapterLecture

Allprimaryresearchentailssomeformofsampling-wherearesearcherselectspeopleorobjectsfromapopulationofinterestforfurtherstudy.

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I.SamplingDefined

Samplingisacommonprocess.Examplesinclude:(1)whenyoulistentoafewtracksfromanewbandinordertodecideifyouwanttohearmore,or(2)whenyoureadseveralmoviereviews(fromamongallthemoviereviewsites)inordertodecideifyouwanttoseeit.

Theseeventscharacterizesampling:toselectandexaminemembersofalarger

populationinordertolearnsomethingnewand,inmostcases,todrawconclusionsaboutthelargerpopulationofwhichthesampleditemsaremembers.

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II.RandomSampling

Mostassociatedwithquantitativeresearch.

Definition:arandomsampleisonewheretheresearcherensuresthateachmemberofthepopulationofinteresthasanequalprobabilityofbeingselected.

Firstpartoflecturefocusesonstepsrelatedtodecisionsinrandom(orprobability)sampling.Secondpartoflecturefocusonnonprobabilitysampling.

A.SampleorCensus

Firststepintherandomsamplingprocessdetermineswhethertousesampleoracensus.

• Sampleexaminesasubsetofthepopulation.

• Censusexamineseverymemberofthepopulation.

Sample,ratherthancensus,isusedinvastmajorityofresearchsituations.Primaryreasons:(1)timeandcostinvolvedinacensus,(2)feasibility-theremaybetoomanypeopleinpopulationforalltobeinterviewed,(3)logistics–maybeimpossibletoreachallmembersofpopulation.

Censusispreferablewhen:(1)populationofinterestissmallandidentifiable,(2)samplingmighteliminateimportantcasesfromthestudy,or(3)credibilityrequirestheconsiderationofallmembersofthetargetpopulation.

B.DefineTargetPopulation

Theneedtoexplicitlyspecifycharacteristicsofthegroupofindividualsorthingsinwhichyouareinterested.Definitionmust:(1)unambiguouslydescribethegroupofinterestand(2)differentiatethosethingsorindividualswhoareofinterestfromthosewhoarenot.

1.TargetDefinitionAndAnInanimatePopulation

Considerproposedtargetdefinition:"ToobtainarandomsampleoftheretailpricesoftheleadingbrandsofpainrelieversinstoresacrosstheUnitedStates."Problemswithproposeddefinition:

• Retailprice-Doesthisrefertotheregularsellingpriceorasaleprice?

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• Leadingbrands-Onwhatbasiswillleadingbrandsbeidentifiedandselected?

• StoresacrosstheUnitedStates-Whattypesofstoreswillbesampled?Wherearetheyspecificallylocated?

• Painrelievers-Anyproductdesignedtorelieveanytypeofpain?

Thefollowingfixestheambiguity:

• Pricereferstotheusualsellingprice.

• Painrelieversrefertoanyaspirin,acetaminophen-oribuprofen-basedproductspecificallysoldtorelieveheadacheormusclepain.

• Leadingbrandsaredefinedasthefivetopsellingbrandsofpainrelievers(asdefinedpreviously)basedon2010unitsales.

• Groceryanddrugstoreswillbethevenueforresearch.

• Theresearchwilltakeplaceintwelvecities.Twelvecitiesrefertofourcitiesamongthetoptencitiesasmeasuredinthe2010populationcensus,fourcitiesamongthoserankedeleventotwenty,andfourcitiesamongthoserankedtwenty-onetothirty.

• Finally,pricesofthefollowingsizedpackageswillbesampled:

100and250regulartabletandthirtytabletgelcap.Thesearethe

leadingsizesbasedonunitsales.

2.TargetDefinitionandAHumanPopulation

Populationsofpeoplearetypicallydefinedinsomecombinationofdemographic,geographic,andbehavioralcriteria.

Considerfollowinghypotheticalcircumstance:ImaginethatwithinthepastfourmonthsMarvelComicshasaddedfiveadditionalpagesofadvertisingtoitsseriesofX-Mencomics.Thesepageshavebeenaddedonatestbasisandwereonlyincludedincomicssenttoretailoutletsandsubscribinghomesthat

arewithintheChicagocitylimits.Marvelnowwantstoexplorereactionstothe

advertising.Fiveresearcherseachpresentadifferentdefinitionofthetarget

population:

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SampledefinitionA:PurchasersofMarvelX-Mencomicbooks.

SampledefinitionB:SubscriberstoMarvelX-Mencomics.

DefinitionsAandBaretoobroad.A"purchaser"ofacomicbookisnotnecessarilythereaderofthecomic.Similarly,asubscriberisnotnecessarilythereader.Since,itisthereader'sopinionsthatareofinterestthesedefinitionsareunacceptable.Also,nomentionofgeographicrestrictions.

SampledefinitionC:ReadersofMarvelX-Mencomicbooks.SampledefinitionD:IndividualswhohavereadanyMarvelX-Men

comicbookwithinthepast30days.

DefinitionsCandDarevagueandambiguous.Inthesedefinitions,a"reader"isanyonewhohaseverreadanX-Mencomic.Thisincludesthosewhohavereadthemostrecentissuesandthosewhoreadanissuefouryearsagobutnotsince.Similarly,"anyX-Mencomics

withinthepast30days"doesnotnecessarilymeanthatthecomicreadisoneofthemorerecentissues."AnyX-Mencomic"canrefertoarecentissueoranissuethatistenormoreyearsold.Theterm,"individuals,"isalsovague.Willthesurveyincludemen(only),women(only)orbothmenandwomen?Nomentionofgeographicrestrictions.

SampledefinitionE:Menandwomenbetweentheagesof10and17whohavereadatleastthreenewissuesofMarvelX-Mencomicswithinthepastfourmonths.

DefinitionEsolvesmanyproblemsofthepriordefinitions,butstillhasproblemsofitsown.Thisdefinitiondefinesareaderintermsofrecentreading("lastfourmonths")andthethreeoffourissuecriterionhelpstoassurethatthereaderhasadequateexperiencewiththetestissuessothatreasonableopinionscanbeformed.However,theageboundariesmakethisdefinitionunacceptable.Thissampledefinitionwouldbiasthesurveybecauseolderindividualsmeetingthereadershipcriterionareexcludedfromthestudy.(Theyoungerageboundaryisreasonableaschildrenyoungerthan10areunlikelytobeabletoanswerthesurveyquestions.)

Amoreacceptabletargetpopulationdefinitionmightbe:

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MenandwomenovertheageofsevenwhohavereadatleastthreenewissuesofMarvelX-Mencomicswithinthepastfourmonths.Theseindividualswilleither(a)obtaintheircomicsbysubscriptionandresidewithintheChicagocitylimitsor(b)benon-subscribersbutpurchasetheminimumofthreenewissuesofX-MencomicsallfromretailoutletsfoundwithintheChicagocitylimits.

3.Selectsamplingmethod

Nextstepselectsprobabilityornon-probabilitysample.Selectioninfluencedbyseveralfactors;(1)thetypeofgeneralizationrequired,(2)researcher'sneedtominimizesamplingerror,(3)studytiming,and(4)cost.

Probabilitysamplesletaresearcher(1)estimatesamplingerror,(2)calculatereliability,(3)statisticallydeterminethesamplesizerequiredforaspecifieddegreeofconfidence,and(4)confidentlygeneralizethefindingstothesampleuniverse.

Nonprobabilitysamplesarequickandinexpensivetoobtainbutprovideverylimitedconfidenceingeneralizingthefindingstothesampleuniverse.

4.SampleFrame

Providesthedetailonwheremembersofthetargetpopulationwillcomefrom.Specifiesthemethodusedtoidentifythehouseholds,individuals,orotherelementsspecifiedinthetargetpopulationdefinition.Twoapproaches:(1)constructorobtainalisttorepresentthetargetpopulationor,whenlistsareincompleteorunavailable,(2)specifyaproceduresuchasrandomdigitdialingforidentifyingandcontactingtargetindividuals.

Theadequacyofasampleframeisevaluatedintermsofhowwelltheframerepresentsthetargetpopulation.

• Aperfectsamplingframeisidenticaltothetargetpopulation,thatis,

thesampleframecontainseverypopulationelementonceandonlyonce

andonlypopulationelementsarecontainedinthesamplingframe.

• Anover-registeredsampleframeistoobroad.Asampleframethatcontainsfewerelementsthanthetargetpopulationsuffersfromunder-registration.

• Anunder-registeredsampleframeistoonarrowandexcludeselementsfromthetargetpopulation.

Over-andunder-registrationmustbeaccountedforinordertoreducebias.

III.TypesofProbabilitySampling

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Threemostcommonformsofprobabilitysamplingare:simple,systematic,andstratified.

A.SimpleRandomSamples

Eachmemberofthepopulation(asrepresentedinthesampleframe)hasaknownandequalchanceofbeingselectedforinclusionintheresearch.Whenperformedcorrectly,sampledpopulationsshouldreflectcharacteristicsofatotalpopulation.

Selectionisaccomplishedthrougharandomevent,suchascointossorrandomnumbertable.

B.SystematicRandomSamples

Typicallyprovidesdataidenticaltosimplerandomsampleswiththeaddedadvantageofsimplicity-notableofrandomnumbersorcointossisneededandsamplesizecanbefirmlyspecified.Worksasfollows:(1)countthenumberofelementsonthelist,(2)determinethedesiredsamplesize,(3)computeaskipinterval,(4)selectarandomplaceonthelisttostart,(5)selecteachelementattheappropriateskipinterval.

C.SimpleandSystematicSamples:OnlineSelection

Therearetimesthatthesampleframereflectsasetofbehaviorsand,asaresult,notallindividualsinthesampleframeareknown,forexample,thedesiretorandomlysamplevisitorstoawebsite.Truerandomsamplescanbeobtained

inthesesituations,butonlyifcareisused.Considerthefollowingsituation:

Books-4-Allhasredesigneditshomepageandseekstoobtainvisitors’reactionstothisredesign.Thenewhomepagegoes“live”onlineatnoononMay17.Books-4-Allwishestointerviewarandomsampleofvisitorstothenewpageandwillconductaroundtheclockinterviewingforthenextweek.Books-

4-Allhastwooptionstoconsider.

• Theycanobtainasimplerandomsamplebyusingsoftwarethat“flips”avirtualcoinforeachvisit,invitingthosewhoseflipcomesup“heads”toparticipateintheresearch.

• Theycanobtainasystematicrandomsample,usingsoftwarethatwillsequentiallycountvisits,invitingthepersononevery10thvisittoparticipateintheresearch.

Neitherapproachisatruerandomsamplebecauseitdoesnotaccountforthenumberofsitevisits–thosevisitingmoreoftenhaveagreaterchanceofselection.Thisproblemcanbesolvedbyensuringthateachindividualhasonlyonechancetobeselectedregardlessofthenumberofvisits.

D.StratifiedSamples

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Simpleandsystematicrandomsamplesprovidefewerreliablegeneralizationsaboutthetotalpopulationwhensignificantdifferencesamongpopulationsubgroupsaresuspected.Wheneverthereisasituationwhereitisbelievedthatthereislargevariationindataofinterestduetorespondentcharacteristics,stratifiedrandomsamplingisabetterchoiceversussimpleandsystematicrandomsampling.

Stratifiedsamplingisaccomplishedinafour-stepprocess:

• First,oneormoreclassificationcriteriathatdefinethestrataareidentified.

• Second,eachelementinthesampleframeisassignedtooneandonlyonestratum.

• Third,thetotalsamplesizeisdetermined.

• Fourth,independentrandomsamples(usingeithersimpleorsystematicsamplingmethods)areselectedfromeachstratuminawaythatresultsinthetotalsamplesizebeingachieved.

Proportionatestratifiedsamplingselectsindividualsinproportiontotheirstratum'ssizewithinthetotaltargetpopulation.

Disproportionatestratifiedsamplingselectsapredeterminednumberofelementsfromeachstratumdespitetherelativesizeofthosestrata.

Approachisselectedbasedonanalyticalconsiderations,thatis,thesamplesizerequiredforreliabledataanalysis.Whendisproportionatestratifiedsamplingisused,thedataobtainedfromanindividualstratummustbeweightedtocompensateforstratumsizedifferentialsintheactualsampleuniversebeforetotalsamplefindingsarereported.

IV.SampleSizeinRandomSamples

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A.ConfidenceLevelandConfidenceInterval

Generally,anduptoapointofdiminishingreturns,largersamplespermitgreaterconfidenceinpopulationestimatesandgeneralizations.

Confidenceisexpressedintermsoftheconfidenceintervalandtheconfidencelevel.

• Aconfidenceintervalisanestimate,plusorminus,ofthevalueofthepopulationestimate;itstatestherangeinwhichwebelievethetruepopulationestimatelies.

• Theconfidencelevelisthemathematicalexpressionofconfidencethatthepopulationestimatelieswithintheconfidenceinterval.Forexample,aconfidencelevelof95%meansthatthereisa95%probabilitythatthepopulationestimatefromtheresearchlieswithintheidentifiedconfidenceinterval.

Samplesizeisdeterminedinlightofconfidenceintervalsandconfidencelevels.Greaterprecisionineitherorbothlevelsrequireslargersamplesizes.Thus,themostimportantstepinsamplesizedeterminationoccurswhen

desiredconfidenceintervalandconfidencelevelisexplicitlystatedpriortodata

collection.

Increasesinconfidencedonotincreaseinalinear,one-to-onerelationshipwithincreasesinsamplesize.Therearediminishingreturns;verylargeincreasesinsamplesizeresultinonlysmallincreasesinconfidencelevelanddecreasesintheconfidenceinterval.

Therearetwoapproachestousingconfidenceintervalandconfidenceleveltodeterminerequiredsamplesize.

• Firstapproachisamanualcalculation,whichusesformulaegroundedinstatisticaltheory.

• Secondapproachusesoneofthenumeroussamplesizecalculatorsavailableonline.

B.SampleSizeandNumberofContacts

Desiredsamplesizeisnotthesameasthenumberofcontacts.Notallpeopleselectedtoparticipateinresearchareavailableoragree.Thus,thenumberofcontactsmustbelargerthanthedesiredsamplesize,calculatedas:

Requirednumberofcontacts=DSS÷(ATP*CS)

whereDSSisthedesiredfinalsamplesize,ATPrepresentsanestimateofthepercentageofthetargetpopulationthatwillagreetoparticipate,andCSrepresentsthepercentageofthoseagreeingthatwillprovidecompletesurveyresponses.(SeeSlide40-4forexamplecalculation).

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V.SampleSelectionBiasinProbabilitySamples

Eliminationofbiasisimportant.Onlywithoutbiascanaresearcherconfidentlygeneralizetheresearchresultstothesampledpopulation.

Samplebiasoccurswhenmembersofthepopulationofinterestareselectedinviolationofbasicprincipleofchance,whereeachobservationhasaknownandequalchanceofbeingselectedforinclusioninthesample.Forexample,useoftelephonebooksasthesourceofnumbersforatelephonesurveyisbiasedbecausethosewhodonothavelistednumbersordonothavealandlinearesystematicallyexcludedfromtheresearch.Theseindividualscanneverbeselected.Thisisthereasonforrandomdigitdialing.

A.BiasandOnlinePanels

Manyresearchcompaniesofferonlinepanelsforresearch.Notallpanelsprovideequaldataquality.Criteriaforevaluation“goodness”ofanonlinepanelinclude:

• Invitationonlypanelsarepreferredover“opt-in”panelsbecausethisrecruitmenttechniquehelpstoreduce“self-selection”bias.

• Panelsshouldaggressivelyandcontinuouslyidentify“professionalrespondents”andimmediatelyexpeltheseindividualsfromthepanel.

• Allpanelmembers’demographicandotherdefininginformationshouldbeverified.

• PaneldemographiccompositionshouldbeverifiedandthepanelitselfshouldmirrorthegeneralU.S.adultpopulation.Researchersshouldnothavetoresorttoweightingresultstocompensateforthelackofpanelrepresentativeness.

• Panelresponseratesshouldbemonitoredandbemadeaccessibletoallresearcherscontemplatinguseofthepanel.

Consistentlyunresponsivepanelmembersshouldbeeliminatedfromthepanel.

• Thequalityofinformationprovidedbypanelmembersshouldbecontinuouslymonitored.Panelmembersthatconsistentlyprovidepoorinformation(forexample,byproviding:thesamechoiceforallquestions,obviouslyinconsistentanswersorcompletingsurveysintooshortatimeframe)shouldbeeliminatedfromthepanel.

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VI.NonprobabilitySampling

Someinformationalneedsdonotrequiretheprecisionandgeneralizabilityofprobabilitysamples.Otherneedscannotjustifytheassociatedtimeandexpense.Inthesecases,nonprobabilitysamplingisused.

Majorformsofnonprobabilitysamplingareconvenience,judgment,quota,andsnowball.

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A.ConvenienceSampling

Respondentsareselectedsimplybecausetheyareconvenientandaccessible,forexample,friends,associatesorindividualswalkingdownthestreetorthroughthemall.

Thishasgreatpotentialtoprovideunreliableandbiasedinformationandshouldonlybeusedwhenthereisabsolutelynoneedtogeneralizetheattitudesandbehaviorsoftheconveniencesampletothebroaderpopulation.Thereisneveranyassurancethatthecharacteristicsoftheconveniencesampleareinanywayrepresentativeofthelargeruniverse.

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B.JudgmentSampling

Individualsareselectedfromthetargetpopulationbasedonanexpert'sjudgmentofwhomightbebesttointerview.Confidenceintheresultsofresearchconductedwithjudgmentsamplesisdirectlyproportionatetotheexpert’sexpertiseusedtoidentifyandselectthesample.Thegreatertheexpertisethemorelikelytheresultscanbetrusted,althoughgeneralizationtothebroaderpopulationofinterestalwaysremainsaproblem.

Approachesincludesamplingof:(1)Extremeordeviantcases,(2)Typicalcaseswhere“average”or“typical”individualsareselected,(3)Highlyintenseorpassionateindividualswhomaynotbeextremeintheirbehaviorsbutarehighlyinvolvedintheareabeingexplored,and(4)Confirmingordisconfirmingcaseswheretheattitudesorbehaviorsofindividualsselected

eithersupportornegatetheresearcher’spre-existingperspective.

Thisisrecommendedonlywhenthereisabsoluteconfidenceintheexpert'sopinionor,similartoconveniencesampling,whenonlypreliminary,exploratoryinformationisrequired.

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C.QuotaSampling

Anextendedformofjudgmentsampling.Attemptstoensurethatdemographicorothercharacteristicsofinterestaresufficientlyrepresentedinthefinalsample.

D.SnowballSampling

Usescurrentstudyparticipantstohelprecruitfutureparticipantsfromamongtheirfriendsandacquaintances.Typicallyusedforverysmall,hardtoreach,orhighlyspecializedpopulationsofindividuals,orpopulationswhereaccessisfacilitatedthroughpersonalintroductions.

VII.SampleSizeandSelectioninNonprobabilitySamples

Unaidedjudgmentisthemostarbitraryapproachtononprobabilitysamplesizedeterminationandshouldbeavoided.

WhatIstheBudget?Theamountoffundsavailableforsamplingisdividedbythecostpersampleunit(forexample,thecosttointerviewoneindividual)andtheresultisusedtosetthesamplesize.Thisshouldalsobeavoided.Buyingthelargestsamplesizethatyoucanaffordhasahighpotentialtoproducesamplesthatareeithertoolargeortoosmallgiventheresearch'sinformationalneeds.

Frameofreferenceisamorereasonableapproachtononprobabilitysamplesizedetermination,wheresampledecisionsfollowthepracticesofothers.

Analyticalrequirementsareprobablybestmethodfordeterminingnonprobabilitysamplesize.Itisrecommendedthatthetotalnumberofindividualsorobservationsinmajorstudysubgroupstotalatleast100whiletherebeaminimumoftwentytofiftyindividualsinminoranalyticalgroups.

ResponsestoApplicationExercises

1.Thesampleuniverseisdefinedasanydonorwithinthepastyear.Thelistinthechairman’sfilesshouldbeignored.Itisunlikelythatthislist(asthesampleframe)showsanyreasonable

correspondencewiththesampleuniverse.Thelistofall11,000donorsshouldbeused,asthisprovidesperfectcorrespondencewiththesampleuniverse.

Disproportionatestratifiedsamplingshouldlikelybeused,asatotalsamplesizeof1,000(forexample)islikelytoincludeonlyabout50“GoldenClub”individualsifsimpleorsystematicrandomsamplingisused.Asampleof973i