$ERROR and LOGIT

Hello, NONMEM has the following property related to intra-subject variability: "During estimation by the first-order method, the exponential model and proportional models give identical results, i.e., NONMEM cannot distinguish between them." So, NONMEM transforms F*DEXP(ERR(1)) into F + F*ERR(1). Is there an easy around it? I try to code the logit transformation. I cannot log-transform the original data as it is suggested in some publications including the presentation by Plan and Karlsson (Uppsala) because many values will be equal to plus or minus infinity. Will NONMEM "linearize" the following code: Z = DLOG((F+THETA(10))/(1-F+THETA(10))) Y = DEXP(Z + ERR(1))/(1 + DEXP(Z + ERR(1))) Thanks! Pavel