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I'd like to fit a mixed-effects logistic growth curve to grouped data using the nlme package in R. The function has the following form:

$$y_t = \dfrac{Ay_0}{y_0+(A-y_0)e^{-rt}} $$

where the parameters $A$ (asymptote), $r$ (rate) and $y_0$ (initial value) are allowed to vary by group. Such a model can be fitted with nlme as follows:

grpdat <- groupedData(y ~ time | group, data=mydata)

logist <- deriv( ~A*y0/(y0+(A-y0)*exp(-r*t)),
                 c("A", "r", "y0"), 
                 function(t, A, r, y0){} )

# without random effects:
mod0 <- nlsList(model = y ~ logist(time, A, r, y0), 
                data = grpdat,
                start = list(A = 100, r = 1, y0 = 0.1)) 

# with random effects:
mod1 <- nlme(mod0, random = list(A ~ 1, r ~ 1, y0 ~ 1))

My question: I have actually observed the values of $y_0$ for each group. How can I specify these, rather than estimate them?

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