[R] Modelling compound logistic growth curves

[Dan Bebber]

Motivated by the discovery of 'loglet analysis'
(http://phe.rockefeller.edu/LogletLab/) that allows one to decompose growth
curves into a series of logistic equations, I attempted to do the same thing
in R.

SIMULATED DATA
Time <- 1:200
pop.size <- SSlogis(Time,10,20,5) + SSlogis(Time,20,100,20) +
rnorm(length(Time))

MY ANALYSIS
results <- nls(size ~ SSlogis(Time, Asym1, xmid1, scal1) + SSlogis(Time,
Asym2, xmid2, scal2),
start = list(Asym1=5, xmid1=15, scal1=30, Asym2=25, xmid2=67, scal2=25))

THE RESULT
I get the error message:
Error in nls(size ~ SSlogis(Time, Asym1, xmid1, scal1) + SSlogis(Time,  :
        step factor 0.000488281 reduced below `minFactor' of 0.000976563

Assistance in doing this analysis would be much appreciated.

Dan Bebber
____________________________
Dr. Daniel P. Bebber
Department of Plant Sciences
University of Oxford
South Parks Road
Oxford
OX1 3RB
Tel. 01865 275060
Web. http://www.forestecology.co.uk/

"Data, data, data!" he cried impatiently. "I can't make bricks without
clay"
- Sherlock Holmes, The Adventure of the Copper Beeches, 1892