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1、Imbens/Wooldridge,LectureNotes1,Summer’071What’sNewinEconometricsNBER,Summer2007Lecture1,Monday,July30th,9.00-10.30amEstimationofAverageTreatmentEffectsUnderUnconfoundedness1.IntroductionInthislecturewelookatseveralmethodsforestimatingaverageeffectsofapr
2、ogram,treatment,orregime,underunconfoundedness.Thesettingisonewithabinaryprogram.Thetraditionalexampleineconomicsisthatofalabormarketprogramwheresomeindivid-ualsreceivetrainingandothersdonot,andinterestisinsomemeasureoftheeffectivenessofthetraining.Unco
3、nfoundedness,atermcoinedbyRubin(1990),referstothecasewhere(non-parametrically)adjustingfordifferencesinafixedsetofcovariatesremovesbiasesincomparisonsbetweentreatedandcontrolunits,thusallowingforacausalinterpretationofthoseadjusteddifferences.Thisisperhap
4、sthemostimportantspecialcaseforestimatingaveragetreatmenteffectsinpractice.Alternativestypicallyinvolvesstrongassumptionslink-ingunobservablestoobservablesinspecificwaysinordertoallowadjustingfortherelevantdifferencesinunobservedvariables.Anexampleofsucha
5、strategyisinstrumentalvariables,whichwillbediscussedinLecture3.AsecondexamplethatdoesnotinvolveadditionalassumptionsistheboundsapproachdevelopedbyManski(1990,2003).Underthespecificassumptionswemakeinthissetting,thepopulationaveragetreat-√menteffectcanbee
6、stimatedatthestandardparametricNratewithoutfunctionalformassumptions.Avarietyofestimators,atfirstsightquitedifferent,havebeenproposedforimplementingthis.Theestimatorsincluderegressionestimators,propensityscorebasedes-timatorsandmatchingestimators.Manyoft
7、heseareusedinpractice,althoughrarelyisthischoicemotivatedbyprincipledarguments.Inpracticethedifferencesbetweentheesti-matorsarerelativelyminorwhenappliedappropriately,althoughmatchingincombinationwithregressionisgenerallymorerobustandisprobablytherecomm
8、endedchoice.Moreim-portantthanthechoiceofestimatoraretwootherissues.Bothinvolveanalysesofthedatawithouttheoutcomevariable.First,oneshouldcarefullychecktheextentoftheoverlapImbens/Wooldridge,LectureNotes1,Summer’072incovariatedistributio
