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st: RE: Re: Skilmack across group


From   "Lam, Chun Nok" <ChunNok.Lam@med.usc.edu>
To   "statalist@hsphsun2.harvard.edu" <statalist@hsphsun2.harvard.edu>
Subject   st: RE: Re: Skilmack across group
Date   Thu, 9 Aug 2012 17:02:35 +0000

Thank you very much Joseph. Let me give a better picture of the data. 

I have 2 groups, intervention and control. And I measured subjects' HbA1c at baseline and follow-up:
                                 Baseline      Follow-up
Intervention           I1                      I2 
Control                     C2                     C2

I want to find out if the change of HbA1c from baseline to follow-up is significantly different cross groups.
We have a small sample, and HbA1C is not normally distributed; therefore I think a nonparametric test would be appropriate. 

The reason I looked into -skilmack- was because 1) nonparametric, 2) repeated measure, and 3) we had lost to follow-up (data structure is unbalanced). But you are right, it might not be designed to compare two treatment groups like ANOVA. Since you suggests there could be a resampling method to get to this with -skilmack-, could you please guide me to that?

In addition, I saw a study (http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2758650/) used another test called van Elteren test -vanelteren-. This is what the study describe their use, which is very similar to what I hope to obtain: "The van Elteren test was used to assess differences in symptom scores of patients on propranolol and placebo stratified by time since baseline evaluation." However, I am unsure how to use the test in the right way. So I tried by making my data in long form and run:

vanelteren hba1c, by(group) strata(survey)

And my data is like: (group = 0 is control) 

Id    survey    group   hba1c
1          1              0            9.6
1          2              0            7.0           
2          1              1            8.5 
2          2              1            8.4
....

Since there is no id() nor repeated() in -vanelteren- like those in -skilmack- or -anova-, I am not sure how this test work in repeated measure and if the stratification is appropriate for that. 

Sorry for the long explanation. And thank you so much once again for your help and guidance. 

Chun Nok Jonathan Lam
Project Manager
Department of Emergency Medicine
Keck School of Medicine of USC
University of Southern California

-----Original Message-----
From: owner-statalist@hsphsun2.harvard.edu [mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Joseph Coveney
Sent: Thursday, August 09, 2012 4:32 AM
To: statalist@hsphsun2.harvard.edu
Subject: st: Re: Skilmack across group

Jonathan Lam wrote:

I want to compare agreement between k measurements cross groups. I am aware of the non-parametric Skillings-Mack test, which can handle repeated measures with unblanaced observations. However, I don't know how to execute the corss-group comparision (invention vs. control) in this test which is similar to what I did using ANOVA:

For skilmack, this is what I have:
Skilmack hba1c, id(id) repeated(survey)

For repeated measure ANOVA, this is what I have:
Anova hba1c group / id|group survey group#survey, repeated(survey)

Can you show me how to compare cross group with the skilmack test? 

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

Neither the Friedman test nor the Skillings-Mack test is intended to be used with individuals nested within groups, and so you're not going to be able to use them in a split-plot design like for your repeated-measures ANOVA.

I'm not sure that I understand what it is that you want to do.  Do you want to compare the magnitude of inter-rater agreement in Group A to the magnitude of inter-rater agreement in Group B?  That is, are you seeking to use Kendall's coefficient of concordance from -friedman- (or some surrogate of it from
-skilmack-) as a nonparametric index of agreement?  If so, then you might be able to use -skilmack- on the groups separately in conjunction with a resampling method.

Joseph Coveney


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