Hi: Perhaps the self-starting functions may be helpful. See ?selfStart. There are self-starting functions for both the logistic and Gompertz models (SSlogis and SSgompertz, respectively). Go through the examples to see how they work.
HTH, Dennis On Fri, Jun 17, 2011 at 2:14 PM, Sean Bignami <bignam...@gmail.com> wrote: > I am trying to fit a curve to a cumulative mortality curve (logistic) where y > is the cumulative proportion of mortalities, and t is the time in hours (see > below). Asym. at 0 and 1 >> y > [1] 0.00000000 0.04853859 0.08303777 0.15201970 0.40995074 0.46444992 > 0.62862069 0.95885057 1.00000000 > [10] 1.00000000 1.00000000 >> t > [1] 0 13 20 24 37 42 48 61 72 86 90 > > I tried to find starting values for a Gompertz non-linear regression by > converting the equation (y~1*exp(-beta*exp(-gamma*t)) to a linear form per > Dalgaard "Introductory Statistics with R" pg.279-280. But got this Error > message: > >> lm(log(0-log(y))~t) > Error in lm.fit(x, y, offset = offset, singular.ok = singular.ok, ...) : > NA/NaN/Inf in foreign function call (arg 4) > > I tried to change all by 0 and 1 values to non-zero and non-one values (yy > and tt below), and was able to get starting estimates. > >> yy<-c(0.000000001,0.04853859, 0.08303777, 0.15201970, 0.40995074, >> 0.46444992, 0.62862069, 0.95885057, >> 0.9999999999,0.999999999999,0.999999999999) >> tt<-c(0.0000000001,13,20,24,37,42,48,61,72,86,90) > >> lm(log(0-log(yy))~tt) > > Call: > lm(formula = log(0 - log(yy)) ~ tt) > > Coefficients: > (Intercept) tt > 9.5029 -0.3681 > > However, when I plug those values into the nls function, I get an error > message about the "getInitial" method > >> nlsout<-nls(y~1*exp(-beta*exp(-gamma*t),start=c(beta=exp(9.5),gamma=.368)));summary(nlsout) > Error in getInitial.default(func, data, mCall = as.list(match.call(func, : > no 'getInitial' method found for "function" objects > > Can anyone help clarify how I can find the parameters for a best-fit curve > for this data? Thanks!! > > Sean > [[alternative HTML version deleted]] > > ______________________________________________ > 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. > ______________________________________________ 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.