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st: time-effect in manova (anova with repeated measures )


From   "Maren Kandulla" <m.kandulla@uke.uni-hamburg.de>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: time-effect in manova (anova with repeated measures )
Date   Mon, 14 Nov 2005 18:06:03 +0100

Dear statalist,
again I am fighting with stata to do the right analysis. My data is a panel
design, 6 measuring times, 4 groups to compare, n=75.
I try to test whether 4 different groups differ in terms of means over time
regarding a variable (motivation). In other words wether motivation differs
between the 4 groups and wether there is a change over time (motivation is
decreasing f.e.). First I did a nice graph to see the differences but now I
would like to test them using manova.
I did a manova (for repeated measures)
    manova motiva_1 motiva_2 motiva_3 motiva_4 motiva_5 motiva_6 = group
Then I checked, which of the 4 groups differ significantly.
    mat c=(0,1,-1,0,0\0,1,0,-1,0\0,1,0,0,-1)
    manovatest, test(c) ytransform(m)
    mat c1=(0,1,-1,0,0)
    mat c2=(0,1,0,-1,0)
    mat c3=(0,1,0,0,-1)
    mat c4=(0,0,1,-1,0)
    mat c5=(0,0,1,0,-1)
    mat c6=(0,0,0,1,-1)
    manovatest, test(c1) ytransform(m)
    manovatest, test(c2) ytransform(m)
    manovatest, test(c3) ytransform(m)
    manovatest, test(c4) ytransform(m)
    manovatest, test(c5) ytransform(m)
    manovatest, test(c6) ytransform(m)
This worked out perfectly.

1.) However, now I would like to see, wether there is a over-all time-effect
(wether motivation is decreasing or increasing over time) for all groups
together and for each group. This is very easy in SPSS but I did not figure
it out in Stata even though I read different manuals and books.
2.) Secondly I would like to see wether there is a significant difference
between two groups at a certain time-point (f.e. group 2 and 4 at time 3) .
I could do this by using a simple anova but maybe I can integrate it in the
manova-model as well?
3.) Thirdly I would like to know wether a decrease for one group differ from
a certain increase of another group between two time-points (f.e. wether the
decrease of group 4 between time 2 and time 3 differes significantly from
the increase of group 1 at the same time period.

Could you please tell me, how to write the matrix for it. I just did not
understand it by reading the manual or other books. Question 1 is the most
important. I know I could do Question 2 and 3 by using a form of
panelanalysis, but since I am working with psychologist I would rather go
for anova with repeated measures.


Thanks a lot,
Maren

*****************************************************************
Maren Kandulla
Wissenschaftliche Mitarbeiterin / Evaluation
Modellstudiengang Medizin
Universitätsklinikum Hamburg-Eppendorf

Martinistr. 52
20246 Hamburg
Tel.: ++49 40 42803 7582
Fax: ++49 40 42803 6251
Email: m.kandulla@uke.uni-hamburg.de
Homepage: www.uke.uni-hamburg.de/studiengaenge/modellstudiengang/index.php

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