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st: the sign of z statatistic after -ranksum-?


From   gjhxmu@sina.com
To   statalist<statalist@hsphsun2.harvard.edu>
Subject   st: the sign of z statatistic after -ranksum-?
Date   Sun, 25 Oct 2009 11:28:49 +0800

Dear statalists,

I use -ranksum- in the stata to do  the Wilcoxon rank-sum test, which is also known as the Mann-Whitney two-sample statistic.

My typing is as followings,

webuse fuel2,clear
ranksum mpg, by(treat) // the z statistic equals -1.279
replace treat=treat==0
ranksum mpg, by(treat) // the z statistic equals 1.279


My doubt is why the sign of z statistic is opposite. As far as I know, in the computation of z statistic the result should be the same no matter which group is compared with which group.


Any help will be appreciated!


Best regards,

Rose.
 

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