How to add a fixed effect to a four-parameter logistic model in nlmer

Question

I am trying to use nlmer with SSfpl to fit some data using a four parameter logistic function. I can get full compliance for general data using:

 nm.fpl <- nlmer(meanFix ~ SSfpl(Time, A, B, xmid, scal) ~ (scal | Subject), data = dataSubset, start = c(A = 0.2, B = 0.7, xmid = 600, scal = 100)) 

Now I want to add a fixed Condition effect, which has 2 levels inside the theme. I would like to evaluate whether the two conditions are different in terms of any of the 4 parameters (A, B, xmid, scal), but I do not know how to indicate this in this formula. I can pick up the model separately for two subsets (condition A and condition B), and then compare the parameters, but this does not seem to be the right approach.

Answer 1

Have you tried to add interaction?

 nm.fpl <- nlmer(meanFix ~ SSfpl(Time, A * Condition, B, xmid, scal) ~ (scal | Subject), data = dataSubset, start = c(A = 0.2, B = 0.7, xmid = 600, scal = 100)) nm.fpl <- nlmer(meanFix ~ SSfpl(Time, A : Condition, B, xmid, scal) ~ (scal | Subject), data = dataSubset, start = c(A = 0.2, B = 0.7, xmid = 600, scal = 100))