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Multiple Comparisons Emphasizing Selected Contrasts: An Extension and Generalization of Dunnett's Procedure

Juliet Popper Shaffer
Biometrics
Vol. 33, No. 2 (Jun., 1977), pp. 293-303
DOI: 10.2307/2529779
Stable URL: http://www.jstor.org/stable/2529779
Page Count: 11
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Multiple Comparisons Emphasizing Selected Contrasts: An Extension and Generalization of Dunnett's Procedure
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Abstract

Dunnett's procedure for multiple comparisons yields a set of simultaneous confidence intervals for pairwise comparison of the means of each of k - 1 conditions with the mean of a kth condition. As in the case of the Tukey procedure for all pairwise comparisons, the Dunnett procedure can be extended to yield simultaneous confidence intervals for all linear contrasts among the means of the k conditions. The extended confidence intervals are compared with intervals obtained by using the Tukey and Scheffe methods, both for equal sample sizes and for sample size allocation which is near-optimal when comparisons of one condition with the other k - 1 conditions are of primary interest. The extended Dunnett intervals are shorter than intervals obtained by the other methods for such comparisons, and generally longer for other types of contrasts. A method is also presented for generalizing the Dunnett procedure to provide simultaneous confidence intervals for all pairwise intergroup comparisons when a set of means is divided into two groups. This generalized procedure is in turn extended to yield simultaneous confidence intervals for all linear contrasts among the means.

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