Maren Kandulla wrote:
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.
[omitted]
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.
--------------------------------------------------------------------------------
No one else seems to have taken you up, so I'll give it a try.
1.) The design matrix in Stata's -manova- is overparameterized like that in
SAS's PROC GLM or SPSS's GLM. To get the mean of the groups at a given
time, you take _b[_cons] and add the average of the group coefficients.
(See the -lincom- statement in the do-file below). So, to test the time
effect, you need to test this mean across time: the one-row test matrix
will have a one in the constant's column (the first column), and (with equal
group sizes) one-quarter in each of your four groups' columns. This is
illustrated in the do-file below, which uses a dataset from UCLA's Academic
Technology Services' Statistical Computing Resources website. The Web page
is www.ats.ucla.edu/stat/sas/library/comp_repeated.htm and you can compare
Stata's results from the do-file below for the main effects for time (and
the group-by-time interaction) to those in the SAS listing shown on the Web
page. Note that the example dataset from the website has only three
treatment groups, and not four as in your case, so the illustration below
divides the sum of the coefficients by three in order to get the average,
and not four.
2.) A one-row M matrix with a one for the column for Time 3 and zeroes
elsewhere, and then a one-row test matrix with a one for the column for
Group 3 and a minus one for the column for Group 4 and zeroes elsewhere.
(See do-file below.)
3.) Analogous to 2.), a one in the column for Time 2 a minus one for Time 3
and zeroes elsewhere, a one for Group 1 and a minus one for Group 4 and
zeroes elsewhere. (See do-file below--the example dataset has only three
groups, so in the illustration, I substituted Group 3 for your Group 4.)
With 75 people and four groups, you will have unequal representations among
groups. I think that MANOVA does best with equal representation among
groups, just like factorial ANOVA does. At the least, you might need to
adjust the one-quarter to some group-size-weighted fraction in order to get
the -lincom- estimate to match that by -summarize-. I recall B. J. Winer
recommending harmonic means in the context of unbalanced ANOVA in order to
get a better estimate of the cell's contribution to the pooled variance.
This might be worthwhile to look into, depending upon the magnitude of the
imbalance. (Probably not if it's 19, 19, 19 and 18.)
For simple main effects testing involving the repeated measurements in the
context of repeated-measures ANOVA in which the sphericity assumption is on
shakey grounds, David Nichols (see the two postings under the subject
heading "Post-hoc tests" at
http://socsci.colorado.edu/LAB/STATS/SPSS/spss195.html ) suggests using
paired Student t-tests. He mentions that the contrasts can be set up in
SPSS's MANOVA command (which takes advantage of the pooled between-group
error), but it is usually easier to do a paired t-test and accept some loss
of power. In the example below, the p-values were similar (essentially the
same, actually) whether the test was done in -manova- or by -ttest-. The
problem of unbalanced groups goes away with paired Student's t-tests, too.
Joseph Coveney
infile id cond ib1 ib2 ib3 ib4 ib5 ///
ms1 ms2 ms3 ms4 ms5 sr1 sr2 sr3 sr4 sr5 ///
using ///
http://www.ats.ucla.edu/stat/mult_pkg/library/repeat/repeat.txt
compress
tabulate cond
// Three treatment groups, balanced
manova sr1 sr2 sr3 sr4 sr5 = cond
*
* Orientation to -manova-'s parameterization
*
summarize sr1
// Compare that mean to:
lincom [sr1]_b[_cons] + ([sr1]_b[cond[1]] + ///
[sr1]_b[cond[2]] + [sr1]_b[cond[3]]) / 3
// So, in order to track the mean across time,
// the test matrix is 1 1/3 1/3 1/3
*
* Main effects of time
*
matrix M = (1, -1, 0, 0, 0 \ 0, 1, -1, 0, 0 \ ///
0, 0, 1, -1, 0 \ 0, 0, 0, 1, -1)
matrix H = (1, `=1/3', `=1/3', `=1/3')
manovatest , test(H) ytransform(M)
// Cf. SAS results presented in Web page
*
* Group-by-time interaction (lagniappe)
*
matrix H = (0, 1, -1, 0 \ 0, 0, 1, -1)
manovatest , test(H) ytransform(M)
// Cf. SAS results presented in Web page
*
* Group 2 versus Group 3 at Time 3
*
matrix M = (0, 0, 1, 0, 0)
matrix H = (0, 0, 1, -1)
manovatest , test(H) ytransform(M)
ttest sr3 if inlist(cond, 2, 3), by(cond)
*
* Change from Times 2 to 3 between Groups 1 and 3
*
matrix M = (0, 1, -1, 0, 0)
matrix H = (0, 1, 0, -1)
manovatest , test(H) ytransform(M)
generate delta23 = sr2 - sr3
ttest delta23 if inlist(cond, 1, 3), by(cond)
exit
*
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* http://www.stata.com/support/faqs/res/findit.html
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