[R] Modelling compound logistic growth curves

From: Dan Bebber <danbebber_at_forestecology.co.uk>
Date: Thu 29 Jul 2004 - 01:24:31 EST


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



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