ly研究方法专题sampling

SamplingSample&SamplingSample&SamplingWhenasubsetofastudypopulationistobeselectedfordatacollection,theselectionprocessisknownassampling.Whysampling?Constraintsontimeandbudget.GreateraccuracyProbability&NonprobabilitySamplingProbabilitySamplingUserandomprocessesratherthanhumanjudgmentstoselectsample.Probabilitysamplingenhancetheaccuracyandcredibilityofthefindings.Probabilitysampleisastatisticalmodelofthestudypopulation.Probability&NonprobabilitySamplingNonprobabilitySamplingToprovideinformationaboutspecificcasesofthestudypopulationthatareinterestedforthestudy.ResultscanNOTbegeneratetothewholepopulation,why?NonprobabilitySamplingTypeofSamplingConvenienceContrastingCasesTypicalCasesCriticalCasesSnowballQuotaSelectionStrategya)Selectcasesthatarekeyoressentialforoverallacceptanceorassessmentb)Selectcasesbasedontheiravailabilityforthestudyandeaseofdatacollectionc)Groupmembersidentifyadditionalmemberstobeincludedinsampled)Selectcasesthatarejudgedtorepresentverydifferentconditions;oftenwellusedwhenatheoreticallyorpracticallyimportantvariablecanbeusedasthebasisforthecontraste)Interviewersselectsamplethatyieldsthesameproportionsasinthepopulationoneasilyidentifiedvariablesf)SelectcasesthatareknownbeforehandtobeusefulandnottobeextremeNonprobabilitySamplingConvenienceSamplingWhentouse?Whenhighdegreesofinternalvalidityorunbiasedestimatesofaprogram’seffectsareneeded.Canrandomassignmentincreasetheinternalvalidityandtheexternalvalidity?

Internalvaliditycanbeincreasedbutexternalvaliditycannotbeincreased.NonprobabilitySamplingQuotaSamplingHowtouse?Matchthepopulationonobservedcharacteristics.DisadvantagesBiasedSampleSnowballsamplesHowtouse?Recruitafewmembersandaskthemtoidentifyothers.DisadvantagesBiasedSampleProbabilitySamplesRandomSelectionMechanismWhatis?Independentmeansofselectionthatarefreefromhumanjudgmentandotherbiases.Randomselectionprocessunderliesthevalidity,precision,powerandcredibilityofsampledataandstatistics.ProbabilitySamplesPrecisionAppliestothesizeoftheconfidenceintervalaroundaparameterestimatesuchasthemeanorapercentage.PowerReferstotheprobabilityofdetectingadifferenceofaspecifiedsizebetweentwogroupsortwovariables.AFunExampleofPowerOneday,mengottiredofrulingtheearthanddecidedtokilleachotheroutandleavetheworldtowomenRule:Whosehairshorterthan5incheswouldbekilled(tokillallthemen)Results:Someshort-hairedwomenwerekilled.(TypeIerror:refusetheright)Somelong-hairedmensurvived.(TypeIIerror:acceptthewrong)Menwhogotcorrectlykilled---Power(refusethewrong)TotalerrorDefinition?

Thedifferencebetweenthetruepopulationvalueforthetargetpopulationandtheestimatebasedonthesampledata.3Components:

NonsamplingbiasSamplingbiasSamplingvariabilitySourcesofTotalErrorinSamplingDesignSourcesofTotalErrorinSamplingDesignSourcesofTotalErrorinSamplingDesignNonsamplingBiasThedifferencebetweenthetruetargetpopulationvalueandthestudypopulationvalue.PopulationListingBias?Thedefinitionofthestudypopulationmayexcludesomemembersofthetargetpopulation.Nonresponse?researcher’sinabilitytocontactcertainmembersorsometargetmembersrefusetoprovidedata.MeasurementError:usewrongmeasurement.SourcesofTotalErrorinSamplingDesignSamplingBiasDifferencebetweenthestudypopulationvalueandtheexpectedvalueforthesample.Selectionbias&Howtoadjust?notallmembersofthestudypopulationhaveanequalprobabilityofselection.Estimationbias?theaveragecalculatedusinganestimationtechniqueonthesampledoesnotequalthestudypopulationvalue.eg.,Medianisthebiasedestimateofpopulationmean.SourcesofTotalErrorinSamplingDesignSamplingVariabilityStatisticsfromrandomlyselectedsampleswillvaryfromonesampletothenextduetochance.Howdowedealwithit?Enlargesamplesize:EnsureSampleHomogeneityPracticalSamplingDesignFrameworkPresamplingChoices1Whatisthenatureofthestudy---exploratory,developmental,descriptive,orexplanatory?2Whatarethevariablesofgreatestinterest?3Whatisthetargetpopulationforthestudy?4Aresubpopulationimportantforthestudy?5Howwillthedatabecollected?6Issamplingappropriate?SamplingChoices1Whatlistingofthetargetpopulationcanbeusedforthesamplingframe?2Whatistheprecisionorpowerneededforthestudy？3Whatsamplingdesignwillbeused?4Willtheprobabilityofselectionbeequalorunequal?5Howmanyunitswillbeselectedforthesample?Postsamplingchoices1Howcantheimpactofnonresponsebeevaluated?2Isitnecessarytoweightthesampledata?3Whatarethestandarderrorsandrelatedconfidentintervalsforthestudyestimates?PresamplingChoices1Whatisthenatureofthestudy---exploratory,developmental,descriptive,orexplanatory?Exploratoryresearch&Samplingstrategies?Conductedonnewlyemergingsocialissuesorrecentlydevelopedsocialprogram.Samplesshouldcoverawiderangeofheterogeneousgroups.Developmentalresearch&Samplingstrategies?Studiescommissionedfortheorydevelopmentormethodologicaldevelopment.Contrastingcasesofnonprobabilitydesigncouldbeused.PresamplingChoices1Whatisthenatureofthestudy---exploratory,developmental,descriptive,orexplanatory?Descriptiveresearch&Samplingstrategies?Estimatesofpopulationcharacteristics,attributesorattitudesarestudyobjectives.Probabilitysamplingshouldbeused.Focusontheprecisionoftheestimates.Explanatoryresearch&Samplingstrategies?Examinesexpecteddifferencesbetweengroupsorvariables.Probabilitysamplingshouldbeused.Focusonthepowertodetecteffects.PresamplingChoices4Aresubpopulationimportantforthestudy?Whatwillbetheproblemifweignorethesubpopulation?

Samplemaycontainlessenoughsubpopulationmembersforareliableanalysis5Howwillthedatabecollected?Howdowedealwithnonresponsewhenweusemailedquestionnaires?

Adjustthesamplesizeeg.,nonresponserate20%,sampleneedtoreach800,howmanyquestionnairesshouldwemail?Samplingchoices1Whatlistingofthetargetpopulationcanbeusedforthesamplingframe?Whatissamplingframe?

Theoperationaldefinitionofthepopulation.4flawsexistinsamplingframes:OmissionsDuplicationsIneligiblesClusterlistsSamplingchoices3Whatsamplingdesignwillbeused?SimplerandomsamplingSystematicsamplingStratifiedsamplingClustersamplingSimplerandomsamplingDefinition?Equalchance/probabilityofdrawingeachunitProcedureTakesamplingpopulationNeedlistingofallsamplingunits(“samplingframe”)NumberallunitsRandomlydrawunitsSimplerandomsamplingSystematicsamplingDefinition?SelectsamplingunitsatregularintervalsProcedureArrangetheunitsinsomekindofsequenceDividetotalsamplingpopulationbysamplesizetogetaninterval.(eg1200/60=20)Choosearandomstartingpoint(for20,thestartingpointwillbearandomnumberbetween1and20)Selectunitsatregularintervals(inthiscase,every20thunit)SystematicsamplingStratifiedsamplingWhentouse?PopulationwithdistinctsubgroupsProcedureDivide(stratify)samplingframeintohomogeneoussubgroups(strata)e.g.age-group,occupations;DrawrandomsampleineachstratumSamplesameproportionofsubjectsfromeachstratum

SelectingasamplewithprobabilityproportionaltosizeAreaPopulationProportionSamplesizeSamplingsizefraction

Rural300030%

Total10000

Urban700070%1000x0.7=7001000x0.3=300

1000Stratifiedsampling10%10%ClustersamplingPrincipleWholepopulationdividedintogroupse.g.neighbourhoodsRandomselectsomeclustersWithinselectedclusters,allunitsareincludedExample:2-stageclustersamplingSection4Section5Section3Section2Section1Allsecond-stageunitsareincludedinthesampleSelectingasamplingmethodPopulationtobestudiedSize/geographicaldistributionHeterogeneitywithrespecttovariableAvailabilityoflistofsamplingunitsLevelofprecisionrequiredResourcesavailableSamplingchoices4Willtheprobabilityofselectionbeequalorunequal?Whendoweneedunequalprobabilityofselection?

Proportionatesampleinclustersamplingdesign.5Howmanyunitswillbeselectedforthesample?Whatwillhappenifthesamplesizeistoosmall?

Samplemaybebiased,unabletodepictthetargetpopulation.Howtodeterminethesamplesize?

DeterminingthesamplesizeisgenerallyaniterativeprocessPostsamplingChoices1Howcantheimpactofnonresponsebeevaluated?Whencanwesaythenonresponserateisacceptable?

Marginalinfluence,randomized,canbeadjusted.2Isitnecessarytoweightthesampledata?Whenshouldweweightthesampledata?Unequalprobabilitiesofselection,unequalresponseratebetweensubgroups.Thescienceofsophomores

(McNemar1946)

HeavyrelianceuponcollegestudentsasexperimentalsubjectsDavidO.Sears1986,CollegeSophomoresintheLaboratory:InfluencesofaNarrowDataBaseonSocialPsychology'sViewofHumanNatureSophomoreGuineaPigThe“ScienceoftheSophomore”Revisited:fromConjecturetoEmpiricismMICHAELE.GORDONL.ALLENSLADEUniversityofTennessee,KnoxvilleNEALSCHMITTMichiganStateUniversity

Whatisresearchsubject?—Anentityobservedforpurposesofresearch(Wikipedia)SubjectsRecruitmentHowpotentialsubjectsareidentifiedHowandbywhomsubjectsareapproachedaboutparticipationWhenconsentisobtainedinrelationtothestartofthestudyprocedures

BackgroundHeavyrelianceoncollegestudentsDominatingmanyareasofbehavioralresearchBasisforjustifyingnewinstancesofappliedbehavioralresearch—”theuseofundergraduatestudentssubjectsinthisstudyisconsistentwithearlierresearch”(HallandHalls1976)Interchangeabilityofstudentand“realpeople”

PhenomenonConsLackofexternalvalidityQuestionablegeneralizabilityProsTheavailabilityandcostadvantageSocialcontentofsomeresearchisminimalUniquesocialandcharacteristicsarelesslikelytoinfluencetheresearchLess-crystallizedattitudes,less-formulatedsenseofself,strongercognitiveskills,strongertendenciestocomplywithauthority,andmoreunstablepeergrouprelationships(DavidO.Sears1986)

ControversyStudiesGoalsTonotewhetherrelationshipsbetweenindependentanddependentvariablesdifferedwhentestedwithstudentsandnonstudents.Sample23articlespublishedinfivemajorjournalsduringthepasttenyears8paperspublishedpriorto1975andreportedinSchneider’s(1982)literatureresearch.MethodTraditionalvotingmethod(meta-analysiscouldnotbeemployedforavarietyofreasons)ResultsMajorDifferenceMinorDifferenceNoDifferenceStatistical12(55%)4(18%)6(27%)Qualitative3(27%)/8(73%)Maybebiasedinfavorofresultsbasedonlargesamples.

BetweensubjectscomparisonSignificantNonsignificantTotalBelowMedian729SampleSizeAboveMedian549Total1268Median=91unitsofanalysisSummaryofStatisticalSignificanceofStudiesBasedonaMedianSplitofSampleSize

Fisher’sexactprobabilitytestDifferencebetweensubjectswasunrelatedtothesamplesize.

Itwouldbewrongtoassumethatstudentsaretotallyunlikenonstudents.—theimpactofbackgroundvariablesoncognitiveappraisalExplanation:DifferentialsubjectfamiliarityOneoftheimportantbackgroundfactorthathassignificantimpactoncognitiveappraisalApossibleexplanationfortheapparentdiscrepancyintheoverallfindingsofthesetsofstatisticalandqualitativecomparisonsLessbetween-groupdifferencesFamiliarity—commonplace/unfamiliartobothgroupsDiscussionandExplanationForresearchers—Prospectsforcontinueduseofundergraduatesinappliedbehavioralresearch—IdeastoimproveexternalvalidityRelianceupontrainedexperimentalsubjects(Gordon1984)Aclinicaldebriefing(Adair1984)EmployingsubjectswithdemographicandinterestprofilessimilartothenonstudentstowhomresearcherswishtogeneralizePrescriptionsfortheFutureForprofessionalgatekeepersEditorsofjournals—assurethatsubjectpopulationssuitedtotheexperimentaltaskareusedintheofappliedresearchpaperstheypublishedFacultycommittees—areencouragedtoweighcarefullytheappropriatenessofexperimentaltaskinassessingthemeritsofthesisanddissertationproposalsinevaluatingtheresearchrecordofjuniorfacultybeingconsideredforpromotionandtenurePrescriptionsfortheFutureTheCollegeSophomoreasGuineaPig:

SettingtheRecordStraightAimedatdissuadeorganizationalresearchersfromprematurelydismissingthefindingsofstudiesusingstudentsamples.Usefulness:equallimitedgeneralizationTheusefulnessofstudentandnonstudentsamplesinorganizationalresearch.Nonstudentsample

RepresentativeGreatergeneralizationOrganizationalsample:homogeneous,typical,narrowRepresentativenessdoesnotensuregeneralizability.Equallyusefulsourcesofinformationunderlyingorganizationalphenomena.Interpretbetween-groupdifferencesDifferentresponseinstudentandnonstudentsamples:invalidresultsfromstudentssamples?between-groupdifference?PhenomenoninquestioninteractwithsubjectcharacteristicBetween-groupdifferencebeasourceofinformation--requiredistinctlydifferentgroupsofhomogeneoussamples--theory-testingresearchVSappl