I'm trying to run a supposed simple ancova model, controlling for pre-test
values. subjects are placed
in three groups and then tested at three different time points. I would like
to covary for the first time
point, examine the group*covariate interaction and calculate the adjusted
means. I've listed 10
subjects (I really have 32) below (g=group, t=time, y=depvar, x=covariate). g
is a between factor, t is a
repeated factor.
id g t y x
1 1 1 1 1
1 1 2 1 1
1 1 3 1 1
3 2 1 1 1
3 2 2 1 1
3 2 3 1 1
4 3 1 5 5
4 3 2 5 5
4 3 3 1 5
6 2 1 5 5
6 2 2 4 5
6 2 3 1 5
7 3 1 6 6
7 3 3 6 6
7 3 2 6 6
8 1 1 3 3
8 1 2 3 3
8 1 3 3 3
9 1 1 6 6
9 1 2 6 6
9 1 3 6 6
10 1 1 1 1
10 1 2 1 1
10 1 3 1 1
When I run this with the 10 subjects listed above, I can't get the interaction
term b/c the df=0 (see
below)
anova y x g / id | g t g*t x*g, cont(x) rep(t)
Number of obs = 24 R-squared = 0.9126
Root MSE = .983192 Adj R-squared = 0.7990
Source Partial SS df MS F Prob > F
-----------+----------------------------------------------------
Model 100.958333 13 7.76602564 8.03 0.0012
x 37.5 1 37.5 25.05 0.0075
g 2.98197901 2 1.4909895 1.00 0.4457
idg 5.98844884 4 1.49711221
-----------+----------------------------------------------------
t 7.6 2 3.8 3.93 0.0550
g*t 4.91666667 4 1.22916667 1.27 0.3437
x*g 0 0
Residual 9.66666667 10 .966666667
-----------+----------------------------------------------------
Total 110.625 23 4.80978261
when I try the adjust command, I come up with blanks (below)
adjust x, by(g t)
Dependent variable: y Command: anova
Variable left as is: id
Covariate set to mean: x = 3.5
t
g 1 2 3
1
2
3
What is really annoying is the fact that I can use this same design in SPSS
and not have a problem, but I
would really like to use STATA. I've examined all the material on the
listserv I can find about this topic,
which has lead me to this point, but if you have another resource please point
me in the right direction.
*
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