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
*
* For searches and help try:
* http://www.stata.com/help.cgi?search
* http://www.stata.com/support/statalist/faq
* http://www.ats.ucla.edu/stat/stata/
