st: RE: Wilcoxon Matched-Pairs Signed-ranks Test also for groups?

["Lachenbruch, Peter" <Peter.Lachenbruch@oregonstate.edu>]

You didn't mention how the groups are matched.  You can't just do a regression using your binary indicator - you need to know which observation is matched with which one.

If your data are continuous and non-normal, I don't have any issue with using the signed rank test.  If you have a lot of ties - say with categorical variables, then there may be an issue but Stata does apply the correction for ties.  If you have dichotomous responses, you're in a McNemar situation, and if you have categorical matched pairs you need the symmetry command.  I think I saw a response from Roger Newsom on the Somersd command that may do better than symmetry.

Tony

Peter A. Lachenbruch
Department of Public Health
Oregon State University
Corvallis, OR 97330
Phone: 541-737-3832
FAX: 541-737-4001


-----Original Message-----
From: owner-statalist@hsphsun2.harvard.edu [mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Rico Baumann
Sent: Thursday, January 29, 2009 5:41 PM
To: statalist@hsphsun2.harvard.edu
Subject: st: Wilcoxon Matched-Pairs Signed-ranks Test also for groups?

Hi,

I have data with a binary variable (0 and 1) that splits my data set into two matched groups (one group of original observations and one control group), therefore the samples are not independent. 

group1  0  varname1 ...
group2  1  varname1 ...

Since some of the contained variables are not normally distributed I´d like to use a non-parametric test in order to test for the median difference of some variables between the two dependent groups.

The above mentioned signrank -- Equality tests on matched data seems to be the correct test, but unfortunately the syntax doesn´t allow the option by(groupvar), like e.g. ttest or ranksum do.
  signrank varname = exp [if] [in]

Is there any (advanced) solution for that, or do I have to rearrange the data in a spreadsheet outside Stata?

Thanks for help and best, 
Rico
-- 
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Rico Baumann

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