I got starting values as follows: Noting that the minimum data value is .38, I fit the linear model log(y - .37) ~ x to get intercept = -1.8 and slope = -.055. So I used .37, exp(-1.8) and -.055 as the starting values for theta0, theta1, and theta2 in the nonlinear model. This converged without problems.
Cheers, Bert On Sun, Aug 20, 2023 at 10:15 AM Paul Bernal <paulberna...@gmail.com> wrote: > Dear friends, > > This is the dataset I am currently working with: > >dput(mod14data2_random) > structure(list(index = c(14L, 27L, 37L, 33L, 34L, 16L, 7L, 1L, > 39L, 36L, 40L, 19L, 28L, 38L, 32L), y = c(0.44, 0.4, 0.4, 0.4, > 0.4, 0.43, 0.46, 0.49, 0.41, 0.41, 0.38, 0.42, 0.41, 0.4, 0.4 > ), x = c(16, 24, 32, 30, 30, 16, 12, 8, 36, 32, 36, 20, 26, 34, > 28)), row.names = c(NA, -15L), class = "data.frame") > > I did the following to try to fit a nonlinear regression model: > > #First, Procedure to Find Starting (initial) Values For Theta1, Theta2, and > Theta3 > > mymod2 <- y ~ theta1 - theta2*exp(-theta3*x) > > strt2 <- c(theta1 = 1, theta2 = 2, theta3 = 3) > > trysol2<-nlxb(formula=mymod2, data=mod14data2_random, start=strt2, > trace=TRUE) > trysol2 > trysol2$coefficients[[3]] > > #Fitting nonlinear Regression Model Using Starting Values From Previous > Part > nonlinearmod2 <- nls(mymod2, start = list(theta1 = > trysol2$coefficients[[1]], > theta2 = trysol2$coefficients[[2]], > theta3 = trysol2$coefficients[[3]]), data = > mod14data2_random) > > And I got this error: > Error in nlsModel(formula, mf, start, wts, scaleOffset = scOff, nDcentral = > nDcntr) : > singular gradient matrix at initial parameter estimates > > Any idea on how to proceed in this situation? What could I do? > > Kind regards, > Paul > > [[alternative HTML version deleted]] > > ______________________________________________ > R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see > 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. > [[alternative HTML version deleted]] ______________________________________________ R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see 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.