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Analysis of Variance Designs by A. J. Guarino, Lawrence S. Meyers, Glenn Gamst

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Chapter Sixteen Analysis of Covariance

16.1 Experimental and statistical control

The designs we have discussed to this point represent techniques of experimental control in order to reduce error variance in a research study (Kirk, 1995). It is also possible to reduce error variance by employing statistical control using analysis of covariance (ANCOVA). Statistical control is used when we know how participants stand (we know their score) on an additional variable that was not part of or could not readily be incorporated into the experimental design. This additional variable is brought into the data analysis by treating it as a covariate. Although covariates can be categorical or quantitative, we will limit our discussion and data analysis procedures ...

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