(Loud, L75, scal),
    data = OME[OME$Noise == "incoherent",],
    start = c(L75=45, scal=3))

# individual fits for each experiment

aa &lt;- factor(OME$Age)
ab &lt;- 10*OME$ID + unclass(aa)
ac &lt;- unclass(factor(ab))
OME$UID &lt;- as.vector(ac)
OME$UIDn &lt;- OME$UID + 0.1*(OME$Noise == "incoherent")
rm(aa, ab, ac)
OMEi &lt;- OME

library(nlme)
fp2 &lt;- deriv(~ 0.5 + 0.5/(1 + exp(-(x-L75)/2)),
            "L75", function(x,L75) NULL)
dec &lt;- getOption("OutDec")
options(show.error.messages = FALSE, Ou