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1、Chapter12NewPerspectivesonCausalMediationAnalysisXiaoluWangandMichaelE.SobelAbstractSocialandbehavioralscientistshavelongusedpathanalysisandrelatedlinearstructuralequationmodels(SEMs)todecomposeparametersoforderedsystemsofequationsintoÒdirecteffectsÓandÒindirecteffectsÓthroughmed
2、iatingvariables.Thesedecompositionshavebeenusedtoaddresssubstantivequestionsoffundamentalinterest,forexample,howapersonÕssocialbackgroundaffectshis/herearningsthrougheducation.However,ingeneral,theÒdirecteffectsÓandÒindirecteffectsÓdeÞnedinandestimatedfromthesemodelsshouldnotbegi
3、vencausalinterpretations,eveninrandomizedexperiments.Toillustratethis,weÞrstdeÞnevariousdirectandindirecteffectsusingpotentialoutcomesnotationanddiscusssituationswhereaninvestigatormightwanttoconsiderthese.Second,weconsideridentiÞcationoftheseeffects:therequiredidentifyingassumpt
4、ionsaremoreoftenthannotimplausibleforthekindsofdatacollectedandquestionsconsideredinsocialandbehavioralresearch.Third,wepresentotheridentifyingassumptionsthatmightbeusedtoidentifydirectandindirecteffects,andthenbrießydiscussdifferentmethodstoestimatetheseeffects,includingregressi
5、on,instrumentalvariables,marginalstructuralmodels,andweightingmethods.Finally,weintroduceanalternativeapproachtomediation(principalstratiÞcation),deÞneseveralpossibleeffectsofinterest,andbrießydiscussidentifyingassumptionsandestimation.IntroductionSocialandbehavioralscientistshav
6、elongusedSewallWrightÕspathanalysisandrelatedlinearstructuralequationmodelstodecomposethetotaleffectsofcausalvariablesintodirectandindirecteffectsthroughmediatingvariables(onthisusage,see,e.g.,Duncan1966;Sobel1990).Thesedecompositionshavebeenusedtoaddresssubstantivequestionsoffun
7、damentalinterest.HauserandFeatherman(1977),forexample,decomposethetotaleffectofapersonÕssocialbackgroundonhis/herearningsintoanindirecteffectthroughhis/hereducationandanetdirecteffect.Thebasicideaofmediationissimple.ConsiderthepathdiagraminFig.12.1,wherethevariableZisacausal/trea
8、tmentvariable,Misamediator,andYistheoutc
