[R] A question about nlmer in lme4 package

[Patty Haaem]

Dear all,I have studied “Mixed models in R using the lme4 package Part 6: Nonlinear mixed models” by Douglas Bates. In this tutorial, there are some codes to fit nonlinear mixed models for Theoph data. The codes are as fallows: >Th. start <- c(lKe = -2.5, lKa = 0.5 , lCl = -3)> nm1 <- nlmer ( conc ~ SSfol (Dose , Time ,lKe , lKa , lCl) ~+    0+ lKe+lKa+lCl +(0+ lKe| Subject )+(0+ lKa| Subject )+    +(0+ lCl| Subject ), nAGQ =0, Theoph ,+     start = Th.start , verbose = TRUE )
  I want to add a covariate (like age) to CL parameter. How should I modify above codes? what's more, how  are selected initial values (lKe = -2.5, lKa = 0.5 , lCl = -3) ?
Thanks in advanceElham
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