On Mar 22, 2012, at 5:06 PM, Stuart Luppescu wrote:
Hello, This one is very perplexing. I have teacher observation data,
with factors teacher ID, observer ID, component, grade and subject.
When
I do this,
aov(data=ratings.prin.22, rating ~ obsid.f + tid.f + subject.f +
grade.f + comp.f)
I get this:
Terms:
obsid.f tid.f grade.f comp.f Residuals
Sum of Squares 306.23399 221.38173 1.70000 14.52831 279.05780
Deg. of Freedom 74 87 2 9 1406
This looks right.
What about the missing SS for subject.f?
There are about 82 observers so 74 degrees of freedom
for obsid.f is OK. But if I do this (just reordering the terms in the
formula):
aov(data=ratings.prin.22, rating ~ comp.f + grade.f + subject.f +
tid.f + obsid.f)
I get this:
Terms:
comp.f grade.f subject.f tid.f obsid.f Residuals
Sum of Squares 15.0923 23.3968 5.9982 499.3568 0.0000
279.0578
Deg. of Freedom 9 4 4 152 3
1406
Now obsid.f only has 3 degrees of freedom and the sum of squares is
0.00. Could this be due to the unbalanced design?
I would have guessed collinearity.
If someone can explain this to me I would be very grateful.
--
Stuart Luppescu -=- slu .at. ccsr.uchicago.edu
David Winsemius, MD
West Hartford, CT
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