Hi there,
 
I am attempting to analyse data from a crossover study using a mixed
effect Emax model in R using nlme. I am having problems ascertaining how
to fit the model.
 
Subjects are randomly assigned to 1 of 4 treatments in a complete block
design. The response variable of interest is recorded once within each
period. So, subjects have a response for each period.
 
Specifically, I want to fit fixed effects: two continuous baseline
covariates (base1 and base2) and period which has levels 1-4. Subject
will be fitted as a random variable.
 
I have started with the 3 parameter Emax model using nls, with no other
covariates and no random term. The syntax of this I know to be correct.
When I progress to nlme, firstly fitting subject as random, I get
nonsensical answers for Emax and ED50. Even if I was getting reasonable
answers at this point, I am not sure how I specify period, a co-factor
in the model?
 
Hoping someone can help.
 
Many thanks in advance.
 
B Surujbally
 
 

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