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 ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.