Abstract
A nonparametric bootstrap was used to obtain an interval estimate of Pearson’s r, and test the null hypothesis that there was no association between 5th grade students’ positive substance use expectancies and their intentions to not use substances. The students were participating in a substance use prevention program in which the unit of randomization was a public middle school. The bootstrap estimate indicated that expectancies explained 21% of the variability in students’ intentions (r = 0.46, 95% CI = [0.40, 0.50]). This case study illustrates the use of a nonparametric bootstrap with cluster randomized data and the danger posed if outliers are not identified and addressed. Editors’ Strategic Implications: Prevention researchers will benefit from the authors’ detailed description of this nonparametric bootstrap approach for cluster randomized data and their thoughtful discussion of the potential impact of cluster sizes and outliers.
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Acknowledgments
The project described was supported by Grant Number DA005629 awarded by the National Institute On Drug Abuse to The Pennsylvania State University (Grant Recipient), Michael Hecht, Principal Investigator, with Arizona State University as the collaborating subcontractor. The data used in the present study would not have been available had it not been for the dedication of the Drug Resistance Strategies Project team members in Phoenix, Arizona. These researchers are led by Drs. Flavio Marsiglia, Stephen Kulis, and Patricia Dustman. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institute on Drug Abuse or the National Institutes of Health. Finally, we would like to thank Drs. Eric Loken and Michael Rovine for helpful comments and suggestions on the preparation of this article.
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Wagstaff, D.A., Elek, E., Kulis, S. et al. Using a Nonparametric Bootstrap to Obtain a Confidence Interval for Pearson’s r with Cluster Randomized Data: A Case Study. J Primary Prevent 30, 497–512 (2009). https://doi.org/10.1007/s10935-009-0191-y
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DOI: https://doi.org/10.1007/s10935-009-0191-y