Dear All,

We have recently used a double logistic model to quantify migration patterns of moose in Sweden. You can find some more information on the biology in our paper http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2656.2010.01776.x/abstract .

The model consists of two logistic non-linear models, one for the spring migration and one for the autumn migration. We used net squared displacement of GPS collared moose, for which we have data for three years  and both sexes, males and females. The model looks like this

M1<-nlme(moosensd ~ asym/(1+exp((xmidA-jday21March)/scale1)) + (-asym /
                (1 + exp((xmidB-jday21March)/scale2))),
  fixed = list(asym+ xmidA + xmidB + scale1 ~ factor(yearnsd)+factor(sexn),scale2~factor(yearnsd)),
   random = asym ~ 1|mooseID,
  start = c(asym =6490,0,0,0, xmidA =71,0,0,0, xmidB = 254,0,0,0, scale1 =8,0,0,0,
  scale2=15,0,0)
  ,data=datansd)

If you want to see what I mean with double logistic, copy and paste the code below
nsd1<-10000/(1+exp((80-(0:365))/5)) + (-10000)/(1+exp((240-(0:365))/5))
plot(0:365,nsd1,type="l", lty=1, tcl=0.5,ylab=expression(paste('Net squared displacement ',' ', (km^2))),
xlab="Months of the year",cex=2.5,xaxt="n",lwd=2.5)
axis(1,at = c(11,72,133,194,255,317), labels =c("April","June","Aug","Oct","Dec","Feb"),tcl=0.5)

The problem is that it can be quite hard to find the right starting values for the above model. Even for a model without fixed effects it can take a while to find the right starting values, which is difficult to handle when one aims to automate the model fitting.
I have seen the selfstarter model for the logistic model (SSlogis) but I couldn't find one for a double logistic model that we use.
Does anyone know of a function in R that provides starting values for a double logistic model?

Many thanks,
Nils

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Nils Bunnefeld
Postdoctoral Research Associate
Imperial College London
Silwood Park
SL5 7PY, Ascot, UK
www.iccs.org.uk<http://www.iccs.org.uk>
http://fp7hunt.net
Tel: +44 20 7594 2253







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