克隆巴赫(信度)系数(Cronbach39;salpha)

1、克隆巴赫(信度)系数(Cronbachsalpha),是心理或教育测验中最常用的信度评估信度工具。其依据一定公式估量测验内部的一致性，作为信度的指标。克服部分折半法的缺点，是目前社会研究最常使用的信度指标，它是测量一组同义或平行测“总和”的信度。克隆巴赫系数公式a=(n/n-1)(1-啓冬)a为信度系数，门为测验题目数，S2.为每题各被试得分的方差，S2t为所有被试所得总分的方差。一般来说，该系数愈高，即工具的信度愈高。在基础研究中，信度至少应达到0.80才可接受，在探索性研究中，信度只要达到0.70就可接受，介于0.700.98均属高信度,而低于0.35则为低信度，必须予以拒绝。SPSSFA

2、QWhatdoesCronbachsalphamean?Cronbachsalphaisameasureofinternalconsistency,thatis,howcloselyrelatedasetofitemsareasagroup.Ahighvalueofalphaisoftenused(alongwithsubstantiveargumentsandpossiblyotherstatisticalmeasures)asevidencethattheitemsmeasureanunderlying(orlatent)construct.However,ahighalphadoesno

3、timplythatthemeasureisunidimensional.If,inadditiontomeasuringinternalconsistency,youwishtoprovideevidencethatthescaleinquestionisunidimensional,additionalanalysescanbeperformed.Exploratoryfactoranalysisisonemethodofcheckingdimensionality.Technicallyspeaking,Cronbachsalphaisnotastatisticaltest-itisac

4、oefficientofreliability(orconsistency).Cronbachsalphacanbewrittenasafunctionofthenumberoftestitemsandtheaverageinter-correlationamongtheitems.Below,forconceptualpurposes,weshowtheformulaforthestandardizedCronbachsalpha:HereNisequaltothenumberofitems,c-baristheaverageinter-itemcovarianceamongtheitems

5、andv-barequalstheaveragevariance.Onecanseefromthisformulathatifyouincreasethenumberofitems,youincreaseCronbachsalpha.Additionally,iftheaverageinter-itemcorrelationislow,alphawillbelow.Astheaverageinter-itemcorrelationincreases,Cronbachsalphaincreasesaswell(holdingthenumberofitemsconstant).AnexampleL

6、etsworkthroughanexampleofhowtocomputeCronbachsalphausingSPSS,andhowtocheckthedimensionalityofthescaleusingfactoranalysis.Forthisexample,wewilluseadatasetthatcontainsfourtestitems-q1,q2,q3andq4.Youcandownloadthedatasetbyclickingonalpha.sav.TocomputeCronbachsalphaforallfouritems-q1,q2,q3,q4-usethereli

7、abilitycommand:RELIABILITY/VARIABLES=q1q2q3q4.Hereistheresultingoutputfromtheabovesyntax:ReliabilityData.3亡D:docuinentssp55一一alpha_faqspas_faq.savWarningsNoSCALEsubconnmanciwasfound.Scalingwillbedoneonallspecifiedvariables.Scale:ALLVARIABLESCaseProcessingSummaryN%CasesValid60100.0Excludeda0.0Total60

8、100.0a.ListwisedeletionbasedonallvariablesirithEprocedure.ReliabilhyStatisticsGroribachsAlphaMOfIt&ITiE.8394Thealphacoefficientforthefouritemsis.839,suggestingthattheitemshaverelativelyhighinternalconsistency.(Notethatareliabilitycoefficientof.70orhigherisconsideredacceptableinmostsocialscienceresea

9、rchsituations.)Inadditiontocomputingthealphacoefficientofreliability,wemightalsowanttoinvestigatethedimensionalityofthescale.Wecanusethefactorcommandtodothis:FACTOR/VARIABLESq1q2q3q4/FORMATSORTBLANK(.35).Hereistheresultingoutputfromtheabovesyntax:FactorAnalysisCommunalitiesIritiGlExtractioriq11.000.

10、585q21.000721q31.000.685q41.000714ExtractionMethod:PrincipalComponentAnalysis.TotalVarianceExplainedComponentInitialEigenvaluesExtractionSumsofSquaredLoadingsTotal%afVarianceCumulative%Total%ofVarianceCumulaiive%1270667.65467.6542.70667.65467.6542.54113.53181.1853.40010.00691.1914.3528.909100.000ExtractionMethod:PrincipalComponentAnalysisComponentMatrix3Component1q2q4q1.049.845,82S.765ExtractionMethod:PrincipalComponentAnalysis.a.1componentsextracted.LookingatthetablelabeledTotalVarianceExplained,weseethattheeigenvalueforthefirstfactor