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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