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1、7Non-experimentalEvaluations7.1TheProblemofCausalInferenceinNon-experimentalEval-uationsWithoutinvokingtheverynonexperimentalmethodstheyseektoavoid,socialexperimentscannotaddressmanyquestionsofinteresttoresearchersandpolicymakers.Eveniftheycould,suchdataaregenerallynotavailable.Asaresult,analystsmus
2、trelyon“obser-vational”ornon-experimentalmethodstoaddresstheproblemofselectionbiasresultingfromnon-randomparticipationofindividualsinemploymentandtrainingprograms.Inanexperimentalevaluation,informationfromthecontrolgroupisusedto…llinmissingcounterfactualdataforthetreatments.Aswehaveseen,undertheassu
3、mptionsspeci…edinSection5,anexperimentismostsuccessfulingeneratingcertaincounterfactualmeans.Inanon-experimentalevaluation,analystsmustreplacethesemissingdatawithdataonnon-participantsalongwithassumptionsdi¤erentfromthoseinvokedwhenusingthemethodofsocialexperiments.Toillustratethispointandtohighligh
4、tanimportantdistinctionbetweenexperimentalandnonexperimentalsolutionstotheevaluationproblem,considerFigure7.1.Itpresentsamodelofpotentialoutcomesinwhicheachoutcometakesononeoftwopossiblevalues.Fortrainingparticipants,Y1equalsoneiftheindividualisemployedaftercompletingtrainingandequalszerootherwise.F
5、ornon-participantsY0isde…nedsimilarly.Asbefore,D=1forpersonswhoselectintotraining(butwhomaybeexcludedinanexperimentalevaluation)andD=0otherwise.Whenprogramevaluatorshaveaccesstoexperimentaldata,theyobservebothY1andY0(butneverbothatthesametimeforthesameperson)forpersonswhoselectintotraining.Thatis,th
6、eyobservetherowandcolumntotalsfortheD=1table,butnottheproportionofpersonsforwhomD=1whoareineachindividualcell.Forexample,theexperimentalcontrolsenabletheanalysttoestimatetheproportionofthepersonsselectingintotraining(D=1)whowouldnothavebeenemployedintheabsenceoftraining,denotedP0:1,butnottheproporti
7、onofpersonsselectingintotrainingwhowouldnothavebeenemployedeitherwithorwithouttraining,denotedP001.Inordertoestimatethisproportion,werequireanotherassumption,suchasthattrainingdidnotcauseanyonetobenon
