handbook on impact evaluation double difference

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1、5.DoubleDifferenceSummaryDouble-difference(DD)methods,comparedwithpropensityscorematching(PSM),assumethatunobservedheterogeneityinparticipationispresentbutthatsuchfac-torsaretimeinvariant.Withdataonprojectandcontrolobservationsbeforeandaftertheprograminterve

2、ntion,therefore,thisfixedcomponentcanbedifferencedout.SomevariantsoftheDDapproachhavebeenintroducedtoaccountforpoten-tialsourcesofselectionbias.CombiningPSMwithDDmethodscanhelpresolvethisproblem,bymatchingunitsinthecommonsupport.Controllingforinitialareacon-d

3、itionscanalsoresolvenonrandomprogramplacementthatmightbiastheprogrameffect.Whereabaselinemightnotbeavailable,usingatriple-differencemethodwithanentirelyseparatecontrolexperimentafterprogramintervention(thatis,aseparatesetofuntreatedobservations)offersanalter

4、natecalculationoftheprogramsimpact.LearningObjectivesAftercompletingthischapter,thereaderwillbeabletodiscuss■Howtoconstructthedouble-differenceestimate■Howtoaddresspotentialviolationsoftheassumptionoftime-invarianthetero-geneity■Howtoaccountfornonrandomprogr

5、amplacement■WhattodowhenabaselineisnotavailableAddressingSelectionBiasfromaDifferentPerspective:UsingDifferencesasCounterfactualThetwomethodsdiscussedintheearlierchaptersrandomizedevaluationandPSMfocusonvarioussingle-differenceestimatorsthatoftenrequireonlya

6、nappro-priatecross-sectionalsurvey.Thischapternowdiscussesthedouble-differenceestima-tiontechnique,whichtypicallyusespaneldata.Note,however,thatDDcanbeusedonrepeatedcross-sectiondataaswell,aslongasthecompositionofparticipantandcontrolgroupsisfairlystableover

7、time.Inapanelsetting,DDestimationresolvestheproblemofmissingdatabymeasur-ingoutcomesandcovariatesforbothparticipantsandnonparticipantsinpre-and71HandbookonImpactEvaluationpostinterventionperiods.DDessentiallycomparestreatmentandcomparisongroupsintermsofoutco

8、mechangesovertimerelativetotheoutcomesobservedforaprein-terventionbaseline.Thatis,givenatwo-periodsettingwheret=0beforetheprogramTCandt=1afterprogramimplementation,lettingYtandYtbetherespectiveo