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Transcript of Regression Discontinuity
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EITM2011
Chris Berr
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.
II. DiscussionofExemplaryPapers
III. Practitioner sGui e
IV. Stataexamples
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RegressionDiscontinuity
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Im r n R f r n RDmethodswereintroducedbyThistlewaite andCampbell
. .
Recentapplicationsinpoliticsincludeanalysesoftheincumbencyeffect(Lee,2008), electoralcompetitionon
, , ,onlegislatorbehavior(Rehavi nd),thevalueofaseatinthe
legisalture (EggersandHainmueller 2009),theeffectof
Recentimportanttheoreticalworkhasdealtwithidentificationissues(Hahn,Todd,andVanDer Klaauw,2001),
design(McCrary,2008),bandwidthselection(Imbens andKalyanaraman 2011).
,Klaauw(2008),andImbensandLemieux(2008). Todaysdiscussionborrowsfromallofthem
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Re ressionDiscontinuit Basics ThebasicideabehindtheRDdesignisthatassignment
tothetreatmentisdetermined,eithercom letel or
partly,bythevalueofanassignment(orforcing)variable(thecovariateX)beingoneithersideofafixedthreshold. Assignmenttotreatmentbycovariatevalue,assignallunits
withXic totreatment
ofthetreatmentatX= c
RDislikearandomizedexperimentatthecutpoint X= c
TheRDdesignisgenerallyregardedashavingthegreatestinternalvalidityofallquasiexperimentalmethods.Itsexternalvalidityismorelimited,sincetheestimatedtreatmenteffectislocaltothediscontinuity.
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RDScatterplot:PositiveTreatmentEffect
(Y)
Assignment Variable (T)Cutting Point
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RDScatterplot:NoTreatmentEffect
(Y)
Assignment Variable (T)Cutting Point
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Startwiththeusualpotentialoutcomesframework
TheunitleveltreatmenteffectisYi(1) Yi(0),whereYi(1)istheoutcomethatwouldoccuriftheunitwereexposedtotreatment Y 0 if not. The familiar roblem is that we cannotobservethepairYi(0)andYi(1)simultaneously.
AssumeabinarytreatmentvariableTi andacontinuousi
Inasharpregressiondiscontinuitydesign:
=
Wherecisacutoffsuchthatallunitsabovereceivethetreatmentandallthosebelowdonot
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Underfairlyweakassumptions,theeffectofthe
cXYEcXYE iiii |lim|lim00
Whichis: RD =E[Yi(1) Yi(0)|Xi=c].
cc iiii mm00
Thisistheaveragetreatmenteffectatthecutoffpoint,aparticularLATE.
andE[Y(0)|X]are(assumedtobe)continuousinx.
SomeextrapolationisrequiredbecausebydesigntherearenounitswithXi =cforwhomweobserveYi(0).
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RDComparedtoOtherObservationalMethods
Remem ert etwoassumptionsnee e toi enti ytreatmente ectsfromobservationaldatausingregression/matching(Kosukes slide#31):overlap andunconfoundedness. .
Ingeneral,unconfoundedness isnotconsideredaparticularlycredibleassumption,andtheothermethodswerestudyingthisweekcanbethoughtofaswaysformakingitmoreplausible.
RDisspecialinthefollowingways: Unconfoundedness issatisfiedbydefinition.WhenXc,Tisalways1;when
X
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RDComparedtoanExperiment RDisoftendescribedasaclosecousinofarandomizedexperimentorasa
localrandomizedexperiment.
Coughey andSekhon argueagainstthisconceptualization,forreasonswewillsee a er, u swor un ers an ngw y eana ogy sma e
Consideranexperimentinwhicheachparticipantisassignedarandomlygeneratednumber,v,fromauniformdistributionovertherange[0,1]. Unitswithv 0.5assi nedtotreatment unitswithv
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ExperimentasRD
(Y)
Assignment Variable (T)
Cutting Point
0.50
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EstimationBasics1
Wehavenowdefinedacausaleffectasthedifferenceoftwofunctionsatapoint.Howdowe
.
Approach#1:Comparemeans = , ,
arenounitsatthecutoffthatdontgetthetreatment,
butinprincipleitcanbeapproximatedarbitrarilywellby=
Thereforeweestimate:
cXYEcXYE ||
Thisisthedifferenceinmeansforthosejustaboveandbelowthecutoff.
Thisisanon arametrica roach.A reatvirtueisthatitdoesnotdependoncorrectspecificationoffunctional
forms.
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NotethatIsaidinprinciplewecanestimatemeans
arbitraril closetothecutoff.In ractice,thisde endsonhavinglotsofdatawithin ofthecutoff.Supposeyoudont.
TheoriginalRDdesign(Thistlewaite andCampbell1960)was
implementedbyOLS.
where isthecausaleffectofinterestand isanerrorterm.
XTY
Thisregressiondistinguishesthenonlinearanddiscontinuousjumpfromthesmoothlinearfunction.
OLSwithonelinearterminXisseldomusedanymoreecauset e unctiona ormassumptionsareverystrong.
Whatarethey?
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Supposetheunderlyingfunctionsarenonlinearandmaybeunknown.In
particular,supposeyouwanttoestimate
wheref(X)isasmoothnonlinearfunctionofX.
)(XfTY
.CommonpracticeistofitdifferentpolynomialfunctionsoneachsideofthecutoffbyincludinginteractionsbetweenTandX.
Modelingf(X)withapthorderpolynomialinthiswayleadsto
p
p
p
p
TXTXTXT
XXXY
...
...
221
02
0201
coefficientonTisthetreatmenteffect.
Commonpractice,forwhateverreason,seemstousea4th orderpolynomial,thoughyoushouldbesurethatyourresultsarerobusttootherspecificationsmoreont is e ow .
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EstimationBasics4 OLSwithpolynomialsisaparticularlysimplewayofallowinga
flexiblefunctionalforminX.Adrawbackisthatitprovidesglobalestimatesoftheregressionfunctionthatusedatafarfromthe
cutoff. Theremanyareotherways,buttheRDsetupposesacoupleof
. Weareinterestedintheestimateofafunctionataboundarypoint.
(Forwhythisisaproblem,seeHTVorImbens andLemieux.)
Standardnon arametrickernelre ressiondoesnotworkwellhere
ThisleadstoApproach#3:LocalLinearRegression Insteadoflocallyfittingaconstantfunction(e.g.,themean),fitlinear
regressionstoobservationswithinsomebandwidthofthecutoff
Arectangularkernelseemstoworkbest(seeImbens andLemieux),butoptimalbandwidthselectionisanopenquestion
Aseriousdiscussionoflocallinearregressionisbeyondthescopeofthislecture.See forexam le FanandGi bels 1996
But,really,werejusttalkingaboutrunningregressionsondatanear
thecutoff.
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RDPitfall:MistakingNonlinearity
forDiscontinuity
formarepotentiallymoresevereforRDthanforothermethodswearestudyingthisweek
Misspecificationofthefunctionalformmay
generateabiasinthetreatmenteffect emos commons u a o no s ype sw enanunaccountedfornonlinearityintheconditionalmeanfunctionismistakenfora
discontinuity Eachofthe3estimationmethodsdealswiththis
ssue na eren way
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(Y)
False discontinuity
Assignment Variable (T)Cutting Point
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(Y)
Assignment Variable (T)Cutting Point
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LocalLinearRegression
(Y)
Assignment Variable (T)Cutting Point
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CompareMeans
(Y
)
Assignment Variable (T)Cutting Point
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CompareMeans:SmallerBandwidth
(Y
)
Assignment Variable (T)Cutting Point
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Manipulation Ifindividualshavecontrolovertheassignmentvariable,thenwe
shouldexpectthemtosortinto(outof)treatmentiftreatmentisdesirable(undesirable) Thinkofameanstestedincomesupportprogram,oranelection
Thosejustabovethethresholdwillbeamixtureofthosewhowouldpassedandthosewhobarelyfailedwithoutmanipulation.
I in ivi ua s aveprecisecontro overt eassignmentvaria e,wewouldexpectthedensityofXtobezerojustbelowthethresholdbutpositivejustabovethethreshold(assumingthetreatmentisdesirable . McCrary(2008)providesaformaltestformaniupulaiton ofthe
assignmentvariableinanRD.TheideaisthatthemarginaldensityofXshouldbecontinuouswithoutmanipulationandhencewelookfor
.
HowprecisemustthemanipulationmustbeinordertothreatentheRDdesign?SeeLeeandLemieux(2010).
Thismeansthatwhen ourunanRD oumustknowsomethin aboutthemechanismgeneratingtheassignmentvariableandhowsusceptibleitcouldbetomanipulation.
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ExampleofManipulation
n ncomesupportprogram nw c t oseearn ngun er , qua y orsupport
SimulateddatafromMcCrary2008
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Itisimpossibletotestthecontinuityassumptiondirectly,but
Namely,allobservedpredeterminedcharacteristicsshouldhaveidenticaldistributionsoneithersideofthecutoff,inthe, . ,
thereshouldbenodiscontinuitiesintheobservables.
Againthereisananalogytoanexperiment:wecannottest,testtheobservables.Rejectioncallstherandomizationintoquestion.
discontinuityinobservablecovariatesindicatesaviolationofthecontinuityassumption,notaviolationof
, .thisbelow)
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In rinci le,covariatesarenotneededforidentificationinRD,buttheycanhelpreducesamplingvariabilityintheestimatorandimprove
Thisisastandardargumentwhichalsosupports
inclusionofcovariatesinanalysesofrandomizedtrials Addingcovariatesshouldnotaffectthepoint
estimateoftheeffect(verymuch). Ifitdoes,
Thewiderthebandwidththemoreimportantitmaybetoincludecovariates.
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GraphicalinspectionisanintegralpartofanyRDanalysis.
3typesofgraphsshouldalwaysbeproduced,where
1:theoutcome
2:othercovariates : ens yo cases
1shouldshowadiscontinuity;2and3shouldshownodiscontinuity
Ifyoucan'tseethemainresultwithsuchasimplegraph,it'sprobablynotthere
,
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BandwidthSelection
ForLocalLinearRegression Bandwidthselectionrepresentsthefamiliartradeoffbetweenbiasandprecision
Whenthelocalregressionfunctionismoreorlesslinear,thereisntmuchofatradeoffsobandwidthcanbelar er.
Therearetwogeneralmethodsforselectingbandwidth
Adhoc,orsubstantivelyderived(e.g.,electionsbetween4852%areclose) Datadriven
Optimalbandwidthmethods(Imbens andKalyanaraman)
Crossvalidationmethods(LudwigandMiller;Imbens andLemieux)
ForPolynomialRegression Choosingtheorderofthepolynomialisanalogtothechoiceofbandwidth
Twoapproaches UsetheAkaike informationcriterion(AIC)formodelselection:AIC=Nln(2)+2p,where 2 (shouldhavea
a s emeansquare erroro eregress onan p s enum ero mo e parame ers
Selectanaturalsetofbins(asyouwouldforanRDgraph)andaddbindummiestothemodelandtesttheirjointsignificance.Addhigherordertermstothepolynomialuntilthebindummiesarenolonger
jointlysignificant. Thisalsoturnsouttobeatestforthepresenceofdiscontinuitiesintheregressionfunctionatpointsotherthenthe
cutoff,whichyoullwanttodoanyway
Inbothcases Inpractice,youmaywanttofocusonresultsfortheoptimalbandwidth,butit'simportanttotestforlotsofdifferentbandwidths.Thinkoftheoptimalbandwidthonlyasastartingpoint.
Ifresultscriticallydependonaparticularbandwidth,theyarelesscredibleandchoiceof.
Inprinciple,theoptimalbandwidthfortestingdiscontinuitiesincovariatesmaynotbethesameastheoptimalbandwidthforthetreatment. Again,followthepracticeoftestingrobustnessto
variationsinbandwidth.
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FuzzyRD Supposetheprobabilityoftreatmentchangesdiscontinuously
atathreshold,butnotfrom0to1.ThisisasituationforapplyingFRD. Notet att e uzziness inFRDcomes romt ec angein
probabilityoftreatment,notfuzzinessaboutthethreshold InsharpRDdesigns,thejumpintheoutcomeatthecutoffisthe
es ma eo ecausa mpac o e rea men . na es gn,thejumpintheoutcomeisdividedbythejumpinthe
probabilityoftreatmentatthecutofftoproducethelocalWald.(InSRD,thejumpisone,sothedivisionisinconsequential,butthisdemonstratestherelationshipbetweenSRDandFRD).
equivalent(andconceptuallysimilar)toIV(seeMostlyHarmlesssec.6.2)
treatmentbyassignmentvariableshouldshowadiscontinuous
probabilityatthethreshold
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RegressionDiscontinuity
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DoVotersAffectorElectPolicies?
byLee,Moretti,andButler(LMB)
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Motivation:2fundamentallydifferentviewsoftheroleofelections
Convergence:Competitionforvotesdrivescandidatestoseekmiddlegroundpolicies,compromise(medianvotertheorem).
o ersa ec po cyc o ceso po c ans.
Divergence:Votersselectcandidates,whothenenacttheirownpreferredpolicies.Voterselectpolicies.
makecrediblepromisestoimplementpoliciesthatarenotattheirownblisspoint(crediblecommitmentsarefacilitatedbyrepeatinteractions)
ThegoalofthepaperistoexaminewhichphenomenonismoreempiricallyrelevantforUSpolitics,specificallyvotingintheHouse
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Consider2parties,DandR
Rsblisspointis0,Dsblisspointisc(>0)
TheprobabilitythatDwinstheelectionisP
IfDwinselection,policyxisimplemented;ifRwins,yis
P*representstheunderlyingpopularityofpartyD,ortheprobabilitythatDwouldwinifx=candy=0.Anincreasein
*
Whendx*/dP*anddy*/dP*>0,wesaythatvotersaffectcandidatespolicychoices
*
Whendx*/dP*anddy*/dP*=0,wesaythatvotersmerelyelectpoliticianswithfixedpolicies.Thatis,anincreasein
* .
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Therollcallvotingrecordoftherepresentativeinthe
RCt =(1 Dt)yt +Dtxt
t . ,
onlythewinningcandidatespolicyisobservable
Theex ressioncanbetransformedinto:
This simply parameterizes the derivatives from
prior slide as 0.
Italsoallowsanindependenteffectofparty,1.
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EstimatingFrameworkContd
WecannotobserveP*soequation(2)cannot
ButsupposewecouldrandomizeDt.ThenDtwou e n epen en o t an t. en:
Everyt ngun er ne nre can eest mate
fromthedata
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GraphicalEstimateofEquation4
20
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5
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0.50
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atthediscontinuity
representativevotingrecords
Resu tsro usttoa ow ng orvar oussortso
districtheterogeneity
Results(smallaffectcomponent)stableovertime
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RegressionDiscontinuityDesigns
ofProIncumbentBiasinCloseU.S.
HouseRaces
byCaughey andSekhon (CS)
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Closeelectionsarenotlikeotherelections
Strategicpoliticalactorshavestrongincentivestotargettheir
resourceswheretheywillhavethegreatestmarginalimpact Thereisanincumbencyadvantageeveninveryclose
elections Theincumbentwinsdisproportionatelyandhasgreaterfinancial
resources Thisfinding,alongwithothercovariateimbalancesatthe
cutpoint,callsintoquestiontheLMBincumbencyadvantageresultsand,moregenerally,theassumptionthatou comes nc osee ec onsare asgoo asran om yassigned NotethatCScritiqueLee(2008),notLMB,buttheimplications
same
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TheLee(&McCrary)TestsforManipulation
A graph like (A) led Lee, and separately McCrary, to conclude that there is no manipulation.
However, (B) and (C) begin to suggest another story. Remember, the concern is with theincumbent partys vote share, not the Democratic vote share.
Density of the Assignment Variable
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DensityoftheAssignmentVariable
Key Takeaway: The candidate of the incumbent party is about three times more likely to winelection by half a percentage point or less than to lose by a similar margin. The density of thisvariable appears to diverge rather than converge in the neighborhood of the cut-point.
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BasedoncorrectingsomeofLeesdataandaddingsome
newvariables,CSfindimbalanceatthecutoffinthe
following: Democraticmargininthepreviouselection
thepartiesrelativecampaignexpenditures
1stdimensionNOMINATEscoreofthecurrentincumbent
whethertheDemocratic(Republican)candidateisthecurrentincumbent
numberoftermstheDemocrat(Republican)hasservedintheU.S.HouseofRepresentatives
w e er e emocra epu can asmorepo caexperiencethantheRepublican(Democrat)
CongressionalQuarterlysOctoberpredictionofwhich
Covariate Imbalance Graph
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CovariateImbalanceGraph
S iti it t B d idth S l ti
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SensitivitytoBandwidthSelection
P t ti l M h i
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PotentialMechanisms
Notlikelybeoutrightfraud,becausesignificanceoflaggedvoteshareisincreasingovertimeandwebelievepotential
Controloverrecountsdoesnotappeartobethekeybecausetheyrarelyhappenandevenmorerarelychange.
Butwedontneedanexplanationbasedonvotecounting.Differencesbetweenwinnersandlosersinincumbency,
, , resourcesareevidentfarbeforeanyvotesarecast,counted,ormanipulated.
closeexanteandinthosethatwereinfactdecidedbyanarrowmargin.
,expectations,andallelseshouldbebalancedintheclosest
elections.
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Thisisacautionarytale
LMBareverygoodscholars.
Theydidalmosteverythingright Theydonotdoalottojustifyfunctionalformorshowrobustnesstodifferentbandwidths
Remember,theLMBresultsforDemocraticvote(eqn.5)arenotimplicatedinthiscritique.Thisisthebestpartofthepaperanyway,IMHO.
Whatcanyoulearnfromthisexchange: Trytofindproblemsinyourdesignbeforesomeoneelsedoesitforyou
Identifyandcollectaccuratedataontheobservablecovariatesmostlikelytoreveal .
yourdataset. Laggedvaluesofthetreatmentvariablearealwaysagoodidea.Inelections,thepartythat
currentlycontrolstheoffice.
Automatedbandwidthselectionalgorithmsdonotguaranteegoodresults.Theyareustastartingpoint.
ForRDpurposes,whatconstitutesacloseelectionappearstobecloserthanthe4852%bandwidthwidelyuseduptonow.CSgetmostoftheirresultsusing49.550.5%.
GiventhecurrentfetishwithRDinpoliticalscience,understandthatitisnotafactofnaturethatcloseelectionsarerandom.Rememberthiswhenyousee(orsetout
towrite)thenextRDpaperoncloseelections.
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RegressionDiscontinuity
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Thetreatmentisdeterminedatleastin artb theassignmentvariable
Thereisadiscontinuityintheleveloftreatmentatsomecutoffvalueoftheassignmentvariable(selectiononobservablesatthecutpoint)
Unitscannotpreciselymanipulatetheassignmentvariabletoinfluencewhethertheyreceivethe
Othervariablesthataffectthetreatmentdonot
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The strength of the RD design is its internal validity,arguably the strongest of any quasi-experimental design
External validity may be limited Shar RD SRD rovides estimates for the sub o ulation
with X=c, that is those right at the cutoff of the assignmentvariable.
The discontinuit is a wei hted avera e treatment effectwhere weights are proportional to the ex ante likelihoodthat an individuals realization of X will be close to thethreshold.
Fuzzy RD (FRD) restricts the estimates further to compliersat the cutoff (more on this below)
(e.g., treatment homogeneity)
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Therearethree eneralt esofthreatstoanRD
design
1. Othervariableschangediscontinuouslyatthe
Testforjumpsincovariates,includingpretreatment
valuesoftheoutcomeandthetreatment2. Therearediscontinuitiesatothervaluesofthe
assignmentvariable
. an pu a ono eass gnmen var a e Testforcontinuityinthedensityoftheassignment
variableatthecutoff
Steps for Sharp RD Analysis
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StepsforSharpRDAnalysis
1. Graphthedatabycomputingtheaveragevalueoftheoutcomevariableoverasetofbins.
The bin width has to be lar e enou h to have a sufficient amount of recisionsothattheplotslookssmoothoneithersideofthecutoffvalue,butatthesametimesmallenoughtomakethejumparoundthecutoffvalueclear.
2. Estimatethetreatmenteffectbyrunninglinearregressionsonbothsidesof
Witharectangularkernel,thesearejuststandardregressionestimatedwithinabinofwidthhonbothsidesofthecutoffpoint.Notethat: Standarderrorscanbecomputedusingstandardleastsquaremethods(robuststandarderrors).The
o timalbandwidthcanbechosenusin crossvalidationorothermethods.3. Therobustnessoftheresultsshouldbeassessedbyemployingvarious
specificationtests. Lookingatpossiblejumpsinthevalueofothercovariatesatthecutoffpoint
variable Lookingwhethertheaverageoutcomeisdiscontinuousatothervaluesofthe
forcingvariable
v ou v u o w ,w w ou o ov maybeavailable.
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1. Graphtheaverageoutcomesoverasetofbinsasinthe
caseofSRD,butalsographtheprobabilityoftreatment.
2. Estimatethetreatmenteffectusing2SLS,whichisnumericallyequivalenttocomputingtheratiointheestimateofthejump(atthecutoffpoint)intheoutcomevariableoverthejumpinthetreatmentvariable.
Standarderrorscanbecomputedusingtheusual(robust)stan ar errors
Theoptimalbandwidthcanagainbechosenusingoneofthemethodsdiscussedabove.
. ero us nesso eresu scan eassesse us ng evariousspecificationtestsmentionedinthecaseofSRDdesigns.
EvaluatinganRDPaper
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g p
oss y our wn Doestheauthorshowconvincinglythat
Treatmentchan esdiscontinuousl atthecut oint
Outcomeschangediscontinuouslyatthecutpoint
Othercovariatesdonotchangediscontinuouslyatthecutpoint Pretreatmentoutcomesdonotchangeatthecutpoint
ere snoman pu a ono eass gnmen var a e unc ngnear ecutpoint)
Arethebasicresultsevidentfromasimplegraph?
Aretheresultsrobusttodifferentfunctionalformassumptionsabouttheassignmentvariable Forexample,parametricandnonparametricfits,differentbandwidths,etc.
Couldotherpossiblyunobservedtreatmentschangediscontinuouslyat
Forexample,18th
birthdaymarksadiscontinuouschangeineligibilitytovote,butalsoeligibilityfordraft,sentencingasanadult,andlotsofotherthings,whichmayormaynotberelevantdependingontheoutcomeinquestion
Arecasesnearthecutpoint differentfromcasesfarfromthecutpoint inotherways?Dothesedifferencesmakethemmoreorlessrelevantfroma
theoreticalorpolicyperspective?
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RegressionDiscontinuity
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EITM2011
Chris Berr