Recommended effect size statistics for repeated measures designs

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Abstract

Investigators, who are increasingly implored to present and discuss effect size statistics, might comply more often if they understood more clearly what is required. When investigators wish to report effect sizes derived from analyses of variance that include repeated measures, past advice has been problematic. Only recently has a generally useful effect size statistic been proposed for such designs: generalized eta squared (eta2G; Olejnik & Algina, 2003). Here, we present this method, explain that eta2G preferred to eta squared and partial eta squared because it provides comparability across between-subjects and within-subjects designs, show that it can easily be computed from information provided by standard statistical packages, and recommend that investigators provide it routinely in their research reports when appropriate.

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Most cited references 6

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Statistical methods in psychology journals: Guidelines and explanations.

Leland Wilkinson (1999)

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Generalized eta and omega squared statistics: measures of effect size for some common research designs.

Stephen Olejnik, James Algina (2003)

The editorial policies of several prominent educational and psychological journals require that researchers report some measure of effect size along with tests for statistical significance. In analysis of variance contexts, this requirement might be met by using eta squared or omega squared statistics. Current procedures for computing these measures of effect often do not consider the effect that design features of the study have on the size of these statistics. Because research-design features can have a large effect on the estimated proportion of explained variance, the use of partial eta or omega squared can be misleading. The present article provides formulas for computing generalized eta and omega squared statistics, which provide estimates of effect size that are comparable across a variety of research designs.

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The development of symbol-infused joint engagement.

Lauren Adamson, Roger Bakeman, Deborah F Deckner (2004)

Fifty-six children were observed longitudinally from 18 to 30 months of age interacting with their mothers during a Communication Play that contained 8 scenes designed to encourage interacting, requesting, commenting, and narrating. Of primary concern was how often symbols infused the child's states of engagement with people and objects and how experience in such symbol-infused states related to language acquisition. Findings indicate that symbols increasingly infuse joint engagement, and that both the timing and the trajectory vary widely among typically developing toddlers, especially during the last half of the 2nd year. Moreover, variations in amount of symbol-infused supported joint engagement may both be influenced by variations in the onset of language and contribute to differences in language facility at 30 months. Copyright 2004 Society for Research in Child Development, Inc.