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1、21Meta-AnalysisThereisacompellingcasetobemadethatanalystsshouldlookatthewholebodyofevidence,ratherthantryingtounderstandindividualstudiesinisolation.Thesystematicreviewofabodyofevidenceisknownasmeta-analysis.Theideaistodrawtogetheralloftheappropriatestudiesthathaveaddressedthesame
2、question,andcalculateanoveralleffectandanoverallmeasureofuncertainlyforthateffect.Ofcourse,thereareplentyofissueswiththis,amongthemthefollowing:Whatdowemeanby‘thesamequestion’?Whatisanappropriatestudy?Whatmakesastudyinappropriate?Howdifferentcanastudybe,andstillbeworthincludin
3、ginthemeta-analysis?Whatarethepublicationbiasesinthevariousstudies?Aretheeffectsfixedorrandom?Thecentralideaistoremoveasmuchaspossibleofthesubjectivitythatwassuchafeatureofold-fashionednarrativereviews.Inanidealworld,weshouldbeabletoextractfromeverypublishedstudytheexactquestiona
4、ddressed,theeffectsize,thevarianceofthateffect,thereplication,andenoughdetailonthemethodsusedtobeconfidentthatthestudywascomparablewiththeothersthatwehavealreadyincluded.Thenwesimplycalculateaweightedaverageeffectsizeandanappropriateweightedaveragemeasureoftheunreliabilityofthatove
5、ralleffect.Andthatwouldbeit.Iftheeffectisconsistentacrossexperiments,thisgivesconsiderableconfidenceinitsgenerality,andthesizeoftheeffectisausefulmeasureofitsimportance.Ontheotherhand,iftheeffectisfoundtobeinconsistentacrossexperiments,thenitsgeneralityiscalledintoquestion,andfurth
6、erstudiesarerequiredtounderstandwhytheeffectwassovariable.Meta-analysisrequires:clearrulesfortheselectionofstudies;clearrulesfortheomissionofstudies;simpleprotocolsforassigningastatisticalweighttoeachstudy;TheRBook,SecondEdition.MichaelJ.Crawley.©2013JohnWiley&Sons,Ltd.Publishe
7、d2013byJohnWiley&Sons,Ltd.META-ANALYSIS741agreedmethodsforcalculatinganappropriatemeasureofuncertainlyfortheeffect;sufficientdetailintheMethodssectionthatanotheranalystwouldgetexactlythesameresult,andcometoexactlythesameconclusion,iftheyweretorepeattheanalysisusingthesamegroupofs
8、tudies.Ameta-analysisofexistingst
