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14_ANCOVA - Analysis of Covariance(ANCOVA A covariate is...

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1 Analysis of Covariance (ANCOVA) A covariate is included into the ANOVA design, in addition to the dependent and independent variables. A covariate in an experiment represents a variable that is known to affect the dependent variable, but not controlled in the design. Æ Particularly a concern in quasi-experimental design. ANCOVA combines regression analysis and analysis of variance (ANOVA). Any ANOVA design can become an ANCOVA design by the addition of a covariate. Purposes of ANCOVA Reduce bias (Adjust estimates of population means on one or more variables). (See Figure 15.1-2, p.708 in Kirk) Increase power (Reduce experimental error). The magnitude of the reduction in the error term is related to the size of the correlation between the covariate and the dependent variable in the design. Æ The larger this correlation, the greater is the reduction in the error term.
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2 (IQ) (Test Score) At group means At grand mean (Example) • DV: Y (Test Score)
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