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ID:7305635
大小:79.48 KB
页数:6页
时间:2018-02-11
《handbook on impact evaluation conclusions》由会员上传分享,免费在线阅读,更多相关内容在工程资料-天天文库。
1、10.ConclusionsImpactevaluationmethodsexaminewhetherprogrameffectscanbeidentified.Thatis,theyseektounderstandwhetherchangesinsuchoutcomesasconsumptionandhealthcanbeattributedtotheprogramitselfandnottosomeothercause.Thishandbookdescribesmajorquantitativemetho
2、dsthatareprimarilyusedinexpostimpactevalu-ationsofprogramsandpolicies.Italsodiscusseshowdistributionalimpactscanbemeasured,aswellasexanteapproachesthatpredicttheoutcomesofprogramsandmechanismsbywhichprogramsaffecttargetedareas.Randomizedevaluationsseektoid
3、entifyaprogramseffectbyidentifyingagroupofsubjectssharingsimilarobservedcharacteristics(say,acrossincomesandearningopportunities)andassigningthetreatmentrandomlytoasubsetofthisgroup.Thenontreatedsubjectsthenactasacomparisongrouptomimiccounterfactualoutcome
4、s.Thismethodavoidstheproblemofselectionbiasfromunobservedcharacteristics.Randomizedevaluations,however,maynotalwaysbefeasible.Insuchcases,research-ersthenturntoso-callednonexperimentalmethods.Thebasicproblemwithanon-experimentaldesignisthatforthemostpartin
5、dividualsarenotrandomlyassignedtoprograms,andasaresultselectionbiasoccursinassessingtheprogramimpact.Thisbookdiscussesanumberofapproachesthataddressthisproblem.Propensityscorematchingmethods,forexample,attempttoreducebiasbymatchingtreatmentandcon-trolhouse
6、holdsonthebasisofobservablecovariates.Propensityscorematchingmeth-odsthereforeassumethatselectionbiasisbasedonlyonobservedcharacteristicsandcannotaccountforunobservedheterogeneityinparticipation.Morecanbelearnedifoutcomesaretrackedforbothparticipantsandnon
7、partici-pantsoveratimeperiodthatisdeemedsufficienttocaptureanyimpactsoftheinter-vention.Apopularapproachinnonexperimentalevaluationisthedouble-difference(ordifference-in-difference)method,althoughthismethodcanalsobeusedinexperi-mentalapproaches.Thismethodpo
8、stulatesthatifoutcomesforbothparticipantsandnonparticipantsaretrackedoveraperiodoftime,suchtrackingwouldprovideagoodbasisforidentifyingtheprogrameffect.Sowithdoubledifference,theobservedchangesovertimefornonp
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