Dear R - Users
I have some problems fitting a linear mixed effects model using the lme
function (nlme library). A sample data is as shown at the bottom of this mail.
I fit my linear mixed model
using the following R code:
bmr <-lme (outcome~ -1 + as.factor(endpoint)+ as.factor(endpoint):trt,
data=datt,
random=~-1 + as.factor(endpoint) +
as.factor(endpoint):trt|as.factor(Trial),
correlation = corSymm(form=~subject|as.factor(endpoint)),
weights=varIdent (form=~subject|endpoint))
With this code, i want to obtain random effects for each Trial. ALso my
residuals are correlated, and the correlated residuals are heteroscedastic
over endpoint. That is, each endpoint has a different constant variance.
However, I recieve the following error message using the code above:
Error in lme.formula(outcome ~ -1 + as.factor(endpoint) +
as.factor(endpoint):trt, :
Incompatible formulas for groups in "random" and "correlation"
May someone kindly inform me of how to correct my code.
Does this imply that the grouping factor in the correlation formula must be
the same grouping factor in the random formula ? If so, is there a way of
getting pass this restriction ?
In essence, I wish to know how to fit a linear mixed model with correlated
residuals (based on a variable in my model) and to obtain the correlation
matrix.
Best regards
Pryseley
Sample data:
RowNames Trial subject VISUAL0 TRT VISUAL24 VISUAL52 TREAT outcome endpoint
trt
4 1 1003 65 4 65 55 2 0 1 1
8 1 1007 67 1 64 68 2 -3 1 -1
12 2 1110 59 4 53 42 2 -6 1 1
14 2 1111 64 1 72 65 2 8 1 -1
16 2 1112 39 1 37 37 2 -2 1 -1
18 2 1115 59 4 54 58 2 -5 1 1
24 3 1806 46 4 27 24 2 -19 1 1
26 3 1813 31 4 33 48 2 2 1 1
28 3 1815 64 1 67 64 2 3 1 -1
4 1 1003 65 4 65 55 2 -10 -1 1
8 1 1007 67 1 64 68 2 1 -1 -1
12 2 1110 59 4 53 42 2 -17 -1 1
14 2 1111 64 1 72 65 2 1 -1 -1
16 2 1112 39 1 37 37 2 -2 -1 -1
18 2 1115 59 4 54 58 2 -1 -1 1
24 3 1806 46 4 27 24 2 -22 -1 1
26 3 1813 31 4 33 48 2 17 -1 1
28 3 1815 64 1 67 64 2 0 -1 -1
Thanks
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