st: ancova for repeated designs

 From tmmanini To statalist@hsphsun2.harvard.edu Subject st: ancova for repeated designs Date Sat, 14 Aug 2004 18:04:21 -0400

```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)

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
which has lead me to this point, but if you have another resource please point
me in the right direction.

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```