PSY 307 – Statistics for the Behavioral SciencesFisher’s F-Test (ANOVA)Testing Yields in AgricultureANOVATwo Sources of VariabilityF-TestDifference vs ErrorF-RatioSlide 9Testing HypothesesFormulas for FFormula for SStotalHours of Sleep DeprivationFormula for SSbetweenFormula for SSwithinDegrees of FreedomChecking Your WorkCalculating F (Computational)F-DistributionANOVA AssumptionsCautionsPSY 307 – Statistics for the Behavioral SciencesChapter 16 – One-Factor Analysis of Variance (ANOVA)Fisher’s F-Test (ANOVA)Ronald FisherTesting Yields in AgricultureX1X2X1= =ANOVAAnalysis of Variance (ANOVA) – a test of more than two population means.One-Way ANOVA – only one factor or independent variable is manipulated.ANOVA compares two sources of variability.Two Sources of VariabilityTreatment effect – the existence of at least one difference between the population means defined by IV.Between groups variability – variability among subjects receiving different treatments (alternative hypothesis).Within groups variability – variability among subjects who receive the same treatment (null hypothesis).F-TestIf the null hypothesis is true, the numerator and denominator of the F-ratio will be the same.F = random error / random errorIf the null hypothesis is false, the numerator will be greater than the denominator and F > 1.F = random error + treatment effectrandom errorDifference vs ErrorDifference on the top and the error on the bottom:Difference is the variability between the groups, expressed as the sum of the squares for the groups.Error is the variability within all of the subjects treated as one large group.When the difference exceeds the variability, the F-ratio will be large.F-RatioF = MSbetween MSwithinMS = SS dfSS is the sum of the squared differences from the mean.F-RatioF = MSbetween MSwithinMSbetween treats the values of the group means as a data set and calculates the sum of squares for it.MSwithin combines the groups into one large group and calculates the sum of squares for the whole group.Testing HypothesesIf there is a true difference between the groups, the numerator will be larger than the denominator.F will be greater than 1Writing hypotheses:H0: 1 = 2 = 3H1: H0 is falseFormulas for FDescription in words of what is being computed.Definitional formula – uses the SS, described in the Witte textComputational formula – used by Aleks and in examples in class.Formula for SStotalSStotal is the total Sum of the SquaresIt is the sum of the squared deviations of scores around the grand mean.SStotal = ∑(X – Xgrand)2SStotal = ∑(X2 – G2/N)Where G is the grand total and N is its sample sizeHours of Sleep Deprivation0 24 480 3 64 6 82 6 10 Grand Total6 15 24 G = 455492025357632253369)45(3)24(3)15(3)6(2222betweenSS 2210064363694160)10()8()6()6()3()2()4()0(22222222ab oveaboveSSwithinFormula for SSbetweenSSbetween is the between Sum of the SquaresIt is the sum of the squared deviations for group means around the grand mean.SSbetween = n∑(X – Xgrand)2SSbetween = ∑(T2/n – G2/N)Where T is each group’s total and n is each group’s sample sizedefinitioncomputationFormula for SSwithinSSwithin is the within Sum of the SquaresIt is the sum of the squared deviations for scores around the group mean.SSwithin = ∑(X – Xgroup)2SSwithin = ∑X2 – ∑T2/n)Where T is each group’s total and n is each group’s sample sizedefinitioncomputationDegrees of Freedomdftotal = N-1The number of all scores minus 1dfbetween = k-1The number of groups (k) minus 1dfwithin = N-kThe number of all scores minus the number of groups (k)Checking Your WorkThe SStotal = SSbetween + SSwithin.The same is true for the degrees of freedom:dftotal = dfbetween + dfwithinCalculating F (Computational)SSbetween = T2 – G2 n NWhere T is the total for each group and G is the grand totalSSwithin = X2 - T2NSStotal = X2 – G2/NF-DistributionCritical valueCommon – retain nullRare – reject nullLook up F critical value in the F table using df for numerator and denominatorANOVA AssumptionsAssumptions for the F-test are the same as for the t-testUnderlying populations are assumed to be normal with equal variances.Results are still valid with violations of normality if:All sample sizes are close to equalSamples are > 10 per groupOtherwise use a different testCautionsThe ANOVA presented in the text assumes independent samples.With matched samples or repeated measures use a different form of ANOVA.The sample sizes shown in the text are small in order to simplify calculations.Small samples should not be
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