I'm trying to better understand the nlme package and have a few questions.
1.)
Other than using various coding strategies (e.g., dummy coding, effect coding), is
there a way to identify group membership (i.e., treatment) directly? For example, the
following code will fit a two group logistic growth curve (where 'Score' is repeatedly
measured over 'Time' for each of the individuals (ID)):
nlme(Score ~ (ALPHA + Group)/(1+exp(-(GAMMA + Group)*(Time - (BETA+Group)))),
data=LE,
fixed=ALPHA + BETA + GAMMA ~ 1,
random=ALPHA + BETA + GAMMA ~ 1,
groups=~ID,
start = c(ALPHA = 1, BETA = 3.25, GAMMA = 2.5))
Rather than specifying the effect of Group in such a manner, is there a simpler way to
identify group membership in order to test the effect of group differences on the
parameters of the model? I thought (removing the dummy codes and) specifying group
membership by: 'groups=~ID/Groups' might work, but an error is returned. I also
thought specifying group membership by :'fixed=ALPHA + BETA + GAMMA ~ 1' might work,
but an error is also returned.
2.)
When will, that is under what circumstances, will there be something different than
'~1' on the right hand side of the 'fixed' and 'random' specification lines?
3.)
Given that I figure out a way to specify group membership/treatment, how are starting
values for both groups specified? Can the covariance structure also be given starting
values?
Sorry for what might turn out to be simply questions. But, as of yet I've not been
able to understand exactly what is going on. Thanks for any help you might be able to
provide.
Have a good one,
Ken
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