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A Note on Interpretation of the Paired-Samples t Test

Donald W. Zimmerman
Journal of Educational and Behavioral Statistics
Vol. 22, No. 3 (Autumn, 1997), pp. 349-360
Stable URL: http://www.jstor.org/stable/1165289
Page Count: 12
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A Note on Interpretation of the Paired-Samples t Test
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Abstract

Explanations of advantages and disadvantages of paired-samples experimental designs in textbooks in education and psychology frequently overlook the change in the Type I error probability which occurs when an independent-samples t test is performed on correlated observations. This alteration of the significance level can be extreme even if the correlation is small. By comparison, the loss of power of the paired-samples t test on difference scores due to reduction of degrees of freedom, which typically is emphasized, is relatively slight. Although paired-samples designs are appropriate and widely used when there is a natural correspondence or pairing of scores, researchers have not often considered the implications of undetected correlation between supposedly independent samples in the absence of explicit pairing.

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