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ERIC Number: ED436571
Record Type: Non-Journal
Publication Date: 1999-Nov
Pages: 17
Abstractor: N/A
Reference Count: N/A
ISBN: N/A
ISSN: N/A
The Importance of Variance in Statistical Analysis: Don't Throw Out the Baby with the Bathwater.
Peet, Martha W.
This paper analyzes what happens to the effect size of a given dataset when the variance is removed by categorization for the purpose of applying "OVA" methods (analysis of variance, analysis of covariance). The dataset is from a classic study by Holzinger and Swinefors (1939) in which more than 20 ability test were administered to 301 middle school students to discern which abilities determined overall academic performance. One-way analyses of variance (ANOVA) were run with the dichotomized variable as the independent factor to compare the reduction in effect size in the ANOVAs with the effect size of regression run with the intact continuous independent variable. The effect sizes of the ANOVAs that were run with the dichotomized variable were about half of the regression effect sizes run with the unaltered independent variable. Each of the effect sizes after dichotomization would fit into a lower category, resulting in a serious underestimation of the substantive effect. (Contains 3 tables and 16 references.) (SLD)
Publication Type: Reports - Evaluative; Speeches/Meeting Papers
Education Level: N/A
Audience: N/A
Language: English
Sponsor: N/A
Authoring Institution: N/A