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ERIC Number: EJ973387
Record Type: Journal
Publication Date: 2012
Pages: 28
Abstractor: As Provided
Reference Count: 18
ISSN: ISSN-0211-2159
Correcting Two-Sample "z" and "t" Tests for Correlation: An Alternative to One-Sample Tests on Difference Scores
Zimmerman, Donald W.
Psicologica: International Journal of Methodology and Experimental Psychology, v33 n2 p391-418 2012
In order to circumvent the influence of correlation in paired-samples and repeated measures experimental designs, researchers typically perform a one-sample Student "t" test on difference scores. That procedure entails some loss of power, because it employs N - 1 degrees of freedom instead of the 2N - 2 degrees of freedom of the independent-samples "t" test. In the case of non-normal distributions, researchers typically substitute the Wilcoxon signed-ranks test for the one-sample "t" test. The present study explored an alternate strategy, using a modified two-sample "t" test with a correction for correlation, analogous to the "z test for correlated samples" used at one time for paired observations. For non-normal distributions, the same modified "t" test was performed on rank-transformed data. Simulations disclosed that this procedure protects the Type I error rate for moderate and large sample sizes, maintains power for normal distributions and several symmetric non-normal distributions, and substantially increases power for various skewed non-normal distributions. (Contains 9 tables, 8 figures and 1 footnote.)
University of Valencia. Dept. Metodologia, Facultad de Psicologia, Avda. Blasco Ibanez 21, 46010 Valencia, Spain. Tel: +34-96-386-4100; Web site:
Publication Type: Journal Articles; Reports - Research
Education Level: N/A
Audience: N/A
Language: English
Sponsor: N/A
Authoring Institution: N/A