[R] SE estimates for treatment groups from nlme
Katherine A Grieve
grieve at u.washington.edu
Mon Apr 10 19:20:03 CEST 2006
I have looked at that example on p. 373, however it still does not help me to
get the stand errors i need. In my case, with the 18 different groups, I am
combining more than just the (intercept) term with one other row in the Summary
tTable. It is no problem to form linear combinations of the coefficients, but
the SE in the tTable cannot just be combined. Similarly, intervals just gives me
CI for each entry (row) in the tTable but these are not the group estimates
that I am looking for.
An example to illustrate what i mean the rows of the tTable would be labeled
something like this (A=2 levels, B=3 levels, C=3 levels):
Fixed Effects: (Asym~A*B*C)
So, to get an estimate for the groups I form linear combinations e.g.:
Group A2,B1,C1 = x1 + x3 + x5 + x11
I am combining 4 coefficients to get the group mean -- is there a way to
get the corresponding SE. It is definitely not a linear combination of the SE
listed in the tTable.
And, intervals just gives me a CI for each entry (row) in the table, not the
linear combinations which form my groups.
It's a bit of fiddling, but there is an example coming close to what you want
on page 373 of Pinheiro/Bates, and in file \library\nlme\scripts\ch08.R,
section 8.2. And don't forget that you can use "intervals" on nlme-results.
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