UseRs,
I am working on the analysis of green area growth in winter wheat and the
effects of the amount of water on it. I am trying to fit a asymmetric
logistic peak curve to my data as described by Royo et al., Europ. J Agronomy
20 (2004) 419. I want to calculate the maximum green area, maximum growth
rate and senescence for each cultivar in each treatment.
I started by calculating the means of all cultivars in all treatments in each
sampling date and fit this data to the curve using the "nls" function
in the
stats package. I am new to non-linear regression and I am getting the error
described below. After doing some search, it seems that the problem is the
start values of the coefficients and some suggestions were done by
linearizing the data in order to have better starting values. I have no idea
how to do this with my data.
Any help in solving this problem will be appreciated.
Marc.
> sessionInfo()
R version 2.9.2 (2009-08-24)
i486-pc-linux-gnu
locale:
LC_CTYPE=en_US.UTF-8;LC_NUMERIC=C;LC_TIME=en_US.UTF-8;LC_COLLATE=en_US.UTF-8;LC_MONETARY=C;LC_MESSAGES=en_US.UTF-8;LC_PAPER=en_US.UTF-8;LC_NAME=C;LC_ADDRESS=C;LC_TELEPHONE=C;LC_MEASUREMENT=en_US.UTF-8;LC_IDENTIFICATION=C
attached base packages:
[1] stats graphics grDevices utils datasets methods base
other attached packages:
[1] chron_2.3-30
> x
ndays y
1 99 0.4047951
2 112 0.5894659
3 125 0.6570246
4 133 0.7065050
5 139 0.6634155
6 148 0.7051833
7 162 0.6794740
8 169 0.6399054
9 175 0.4850703
10 182 0.2961120
> model3 <- nls(x ~ a +
(b/e)*{(1+exp(ndays+d*log(e)-f)/d)^-((e+1)/e)}*{(exp(ndays+d*log(e)-f)/d)^-(e+1)/e}*(e+1)^{(e+1)/e},
+ data = x,start = list(a = -1, b = 0.5, f=-0.4, d=0.6, e=2)
+ )
Error in nlsModel(formula, mf, start, wts) :
singular gradient matrix at initial parameter estimates