I ordinarily use SPSS, and am new to Stata; this is my
first post to this list. Could anyone refer me to
resources for understanding the matrix command, more
particularly for _laying out the contrast coefficients
depending on the number of levels of the IV and DV in
MANOVA_?
I hope my questions are proper ones for this list and
are not too basic. I am doing a Manova and am most
interested in the time by treatment interaction. I am
baffled by the contrasts described in the worked
example at
http://www.ats.ucla.edu/stat/stata/faq/manovarep.htm
.
Any help in understanding how to write contrasts in
such a situation would be much appreciated. The
online
Stata manual doesn't provide much guidance nor have
other web resources/our university library.
Thank you,
Sandy
Below is a more detailed description of the questions
I have about the worked example, and also about my own
data, in case anyone is inclined/willing to give their
two cents.
WORKED EXAMPLE
input s a y1 y2 y3 y4
1 1 3 4 7 7
2 1 6 5 8 8
3 1 3 4 7 9
4 1 3 3 6 8
5 2 1 2 5 10
6 2 2 3 6 10
7 2 2 4 5 9
8 2 2 3 6 11
end
manova y1 y2 y3 y4 = a
mat ymat = (1,0,0,-1\0,1,0,-1\0,0,1,-1)
mat list ymat
I don't understand how these coefficients measure the
interaction, it looks like they just compare each of
y1 to y3 in turn to y4.
MY DATA
My dataset has three repeat measurements of a number
of variables and several IVs of differing levels.
DVTime1 DVTime2 DVTime3 IV1 IV2
3 4 3 1 1
1 2 4 1 2
2 1 5 1 3
3 4 6 1 1
3 5 4 2 2
2 4 3 2 4
3 1 5 2 2
1 4 4 2 2
3 4 4 2 1
The main analysis I need to do looks at the
interaction of time and IV1, which has 2 levels. I
created a matrix using the following
Mat ymat = (1,0,-1\0,1,-1)
However the output was quite different from the output
from SPSS Manova for the very same data and analysis.
Since I am more comfortable with SPSS, I thought it
prudent to check my work before proceeding too far.
What should the matrix be to look at the time by
treatment interaction?
The reason I am not using SPSS to do these analyses is
that Stata is better suited to my non-normal data.
(I ultimately plan to use the negative binomial option
on the Manova, once I get the bugs worked out.)
Thank you very much for any help you can give.
__________________________________________________
Do You Yahoo!?
Tired of spam? Yahoo! Mail has the best spam protection around
http://mail.yahoo.com
*
* For searches and help try:
* http://www.stata.com/support/faqs/res/findit.html
* http://www.stata.com/support/statalist/faq
* http://www.ats.ucla.edu/stat/stata/
