Ah, I see, thank you both.
Michael Weylandt wrote > > It's a scaling problem: > > If you do this: > > datum <- datum[order(datum$X),] > > with(datum, plot(Y~X)) > with(datum, lines(X, 3400*exp(-1867*X))) > > you'll see that your initial guess is just so far gone that the nls() > optimizer can't handle it. > > If you try a more reasonable initial guess it works fine: > > fit <- nls(Y ~ a*exp(-k * X), datum, start=c(a=3400, k=1.867)) > > Michael > > On Wed, Nov 30, 2011 at 12:14 PM, chuck.01 <CharlieTheBrown77@> > wrote: >> Hello, >> I have data like the following: >> >> datum <- structure(list(Y = c(415.5, 3847.83333325, 1942.833333325, >> 1215.22222233333, >> 950.142857325, 2399.5833335, 804.75, 579.5, 841.708333325, 494.053571425 >> ), X = c(1.081818182, 0.492727273, 0.756363636, 0.896363636, >> 1.518181818, 0.499166667, 1.354545455, 1.61, 1.706363636, 1.063636364 >> )), .Names = c("Y", "X"), row.names = c(NA, -10L), class = "data.frame") >> >> >> with(datum, plot(Y~X)) >> >> As you can see there is a non-linear association between X and Y, and I >> would like to fit an appropriate model. I was thinking an exponential >> decay >> model might work well. >> >> I tried the following (a and k starting values are based off of a lm() >> fit), >> but get an error. >> >> fit <- nls(Y ~ a*exp(-k * X), datum, start=c(a=3400, k=1867)) >> >> Error in nlsModel(formula, mf, start, wts) : >> singular gradient matrix at initial parameter estimates >> >> I have never attempted to fit a non-linear model before, and thus the >> model >> may be inappropriately specified, or it is also possible that I have no >> idea >> what I am doing. >> >> Would someone please offer some advice. >> >> Thanks. >> Chuck >> >> >> -- >> View this message in context: >> http://r.789695.n4.nabble.com/nls-help-tp4123876p4123876.html >> Sent from the R help mailing list archive at Nabble.com. >> >> ______________________________________________ >> R-help@ 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@ 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. > -- View this message in context: http://r.789695.n4.nabble.com/nls-help-tp4123876p4124772.html Sent from the R help mailing list archive at Nabble.com. ______________________________________________ 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.