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st: Re: Question about kwallis p-value


From   "Joseph Coveney" <jcoveney@bigplanet.com>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: Re: Question about kwallis p-value
Date   Thu, 3 Dec 2009 12:06:57 +0900

Mahboobeh Safaeian wrote:

I am using the Kruskal Wallis rank test to see if there are any statistically
significant differences in 3 different groups. I have 2 questions (unrelated):

1) How do I determine whether I use the p-values with ties or without ties?

2) is there a statistical test for medians that can account for multiple
observations per person (kind of like a GEE)?

--------------------------------------------------------------------------------

If you have ties in your response variable, then you'd be better off using the 
with-ties p-value.  You can check for ties with the Stata command -isid-.

sysuse census
kwallis medage, by(region)
isid medage

If you're interested in within-person comparisons, then there's Friedman's test;

there are user-written commands, e.g., -friedman- and -skilmack-.  For 
between-groups testing where there are multiple observations on each person, 
there is van Elteren's test (two groups at a time), again, with a user-written 
command, -vanelteren-.  Another user-written command that could be helpful in 
your case is -somersd-, which allows for a -cluster()- option.  All of these can

be downloaded from SSC (-help ssc- for how).  You can try -permute- with a 
randomized-block ANOVA, too.  As I recall, there's a worked example in the 
documentation for -permute-, which is an official Stata command.

"a statistical test for medians"--many nonparametric tests are tests of location

only when the scale is the same between groups.

Joseph Coveney


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