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RE: st: Re: interpretation


From   "Nick Cox" <n.j.cox@durham.ac.uk>
To   <statalist@hsphsun2.harvard.edu>
Subject   RE: st: Re: interpretation
Date   Wed, 11 Jun 2003 17:14:12 +0100

Judith Abrams
> 
> I believe there is an error in kwallis2. I wrote to the 
> author and he 
> disagreed. The reason for my concern is the null case.  
> According to my 
> understanding, if there are only 2 groups, kwallis and 
> kwallis2 should 
> produce the same pvalues, but they do not. For example, 
> using the auto data
> 
>  kwallis trunk, by(foreign)
> 
> Test: Equality of populations (Kruskal-Wallis test)
> 
>   +---------------------------+
>   |  foreign | Obs | Rank Sum |
>   |----------+-----+----------|
>   | Domestic |  52 |  2218.00 |
>   |  Foreign |  22 |   557.00 |
>   +---------------------------+
> 
> chi-squared =    10.045 with 1 d.f.
> probability =     0.0015
> 
> chi-squared with ties =    10.134 with 1 d.f.
> probability =     0.0015
> 
> **
>  kwallis2 trunk, by(foreign)
> 
> One-way analysis of variance by ranks (Kruskal-Wallis Test)
> 
> foreign  Obs   RankSum  RankMean
> --------------------------------
>   0       52   2218.00     42.65
>   1       22    557.00     25.32
> 
> Chi-squared (uncorrected for ties) =    10.045 with    1 
> d.f. (p = 0.00153)
> Chi-squared (corrected for ties)   =    10.134 with    1 
> d.f. (p = 0.00146)
> 
> Multiple comparisons between groups
> -----------------------------------
> (Adjusted p-value for significance is 0.025000)
> 
> Ho: trunk(foreign==1) = trunk(foreign==2)
>     RankMeans difference =     17.34  Critical value =     10.72
>     Prob = 0.000764 (S)

Sorry, but what's the difference here, apart 
from display format? 

Nick 
n.j.cox@durham.ac.uk 

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