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Biostatistics

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694 CHAPTER 13 NONPARAMETRIC AND DISTRIBUTION-FREE STATISTICS<br />

FIGURE 13.6.1<br />

Mann–Whitney test rejection regions for three sets of hypotheses.<br />

8. Statistical decision. When we enter Table L with n ¼ 15; m ¼ 10, and<br />

a ¼ :05, we find the critical value of w a to be 45. Since 25 < 45, we<br />

reject H 0 .<br />

9. Conclusion. We conclude that M X is smaller than M Y . This leads to the<br />

conclusion that prolonged inhalation of cadmium oxide does reduce the<br />

hemoglobin level.<br />

10. p value. Since 22 < 25 < 30, we have for this test :005 > p >:001.<br />

&<br />

Large-Sample Approximation When either n or m is greater than 20 we<br />

cannot use Appendix Table L to obtain critical values for the Mann–Whitney test. When<br />

this is the case we may compute<br />

T mn=2<br />

z ¼ p ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi<br />

(13.6.2)<br />

nmðn þ m þ 1jÞ=12<br />

and compare the result, for significance, with critical values of the standard normal<br />

distribution.<br />

Mann–Whitney Statistic and the Wilcoxon Statistic As was noted<br />

at the beginning of this section, the Mann–Whitney test is sometimes referred to as the

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