Measures of Effect Size for Comparative Studies: Applications, Interpretations, and Limitations

Although dissatisfaction with the limitations associated with tests for statistical significance has been growing for several decades, applied researchers have continued to rely almost exclusively on these indicators of effect when reporting their findings. To encourage an increased use of alternative measures of effect, the present paper discusses several measures of effect size that might be used in group comparison studies involving univariate and/or multivariate models. For the methods discussed, formulas are presented and data from an experimental study are used to demonstrate the application and interpretation of these indices. The paper concludes with some cautionary notes on the limitations associated with these measures of effect size. Copyright 2000 Academic Press.     

Using Wherry's Adjusted R 2 and Mallow's Cp for Model Selection From All Possible Regressions

Selecting a subset of predictors from a pool of potential predictors continues to be a common problem encountered by applied researchers in education. Because of several limitations associated with stepwise variable selection procedures, the examination of all possible regression solutions has been recommended. The authors evaluated the use of Mallow's C_p and Wherry's adjusted R ² statistics to select a final model from a pool of model solutions. Neither the C_p nor the adjusted R ² statistic correctly identified the underlying regression model any better and was generally worse than the stepwise selection method, which itself was poor. Using any of the model selection procedures studied here resulted in biased estimates of the authentic regression coefficients and underestimation of their standard errors. The use of theory and professional judgment is recommended for the selection of variables in a prediction equation.     

Imagery/Concreteness Attributes of Advance Organizers

The purpose of this study was to examine the attributes of prose passages used in advance organizer studies on an imagery-concreteness scale. Phase I demonstrated significant differences in ratings between organizer, historical, and learning passages using a semantic differential type instrument. A principal components factor analysis of the scale provided evidence of a stable factor structure for the instrument across the three passages. Phase II demonstrated significant differences in retention for the organizer-learning passage combination but no differences when the three passages were examined alone. Results are discussed on the basis of Paivio’s two-process theory, and questions are raised about the role of advance organizers in learning and retention.