This is a repost from R-help. Hopefully someone here will be able to give
me more specific advice. Here is the issue:
Say, for instance you want to model growth in pituitary distance as a
function of age in the Orthodont dataset.
fm1 = lme(distance ~ I(age-8), random = ~ 1 + I(age-8) | Subject, data =
Orthodont)
You notice that there is substantial variability in the intercepts (initial
distance) for people at 8 years, and that
this variability in initial distance is related to growth over time:
R# summary(fm1)
...
Random effects:
Formula: ~1 + I(age - 8) | Subject
Structure: General positive-definite, Log-Cholesky parametrization
StdDev Corr
(Intercept) 1.8866 (Intr)
I(age - 8) 0.2264 0.209
Residual 1.3100
Now 2 questions:
1. With lme, how can you get a fit of the growth model accounting for the
relationship between initial status (intercept) and growth?
Some texts call this latent variable regression or something or other, which
seems to basically boil down to adding the random effects
intercept as a predictor in the growth model. Is this done in lme by simply
adding the intercept results from ranef(fm1) to the model?
This two-step process seems wrong to me for some reason, perhaps because it
seems too simple. Anyone know the proper way to do
this in lme? I have read Pinheiro and Bates, and not seen anything that
would help me with this.
2. In addition, suppose you see that there are significant mean differences
in initial status by Sex:
fm2 = lme(distance ~ I(age-8) + Sex, random = ~ 1 + I(age-8) | Subject, data
= Orthodont)
R# summary(fm2)
Fixed effects: distance ~ I(age - 8) + Sex
Value Std.Error DF t-value p-value
(Intercept) 22.917 0.5134 80 44.64 0.000
I(age - 8) 0.660 0.0713 80 9.27 0.000
SexFemale -2.145 0.7575 25 -2.83 0.009
Along the lines of question #1, how would you get a growth model adjusting
for these Sex differences in initial status? I am looking for something
similar to adjusting for baseline differences between Sexes in ANCOVA. I
know Lord would not approve, but this is just by way of example... Thanks
so much for your help, and this wonderful program Dr. Bates.
- DC
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