Dear Bert, Thank you so much for your kind and valuable feedback. I tried finding the starting values using the approach you mentioned, then did the following to fit the nonlinear regression model: nlregmod2 <- nls(y ~ theta1 - theta2*exp(-theta3*x), start = list(theta1 = 0.37, theta2 = exp(-1.8), theta3 = -0.05538), data=mod14data2_random) However, I got this error: Error in nls(y ~ theta1 - theta2 * exp(-theta3 * x), start = list(theta1 = 0.37, : step factor 0.000488281 reduced below 'minFactor' of 0.000976562 nlregmod2 <- nlxb(y ~ theta1 - theta2*exp(-theta3*x), start = list(theta1 = 0.37, theta2 = exp(-1.8), theta3 = -0.05538), data=mod14data2_random) summary(nlregmod2) Object has try-error or missing parameters nlregmod2 And I get some NA values when retrieving the statistics for the fitted model: residual sumsquares = 0.0022973 on 15 observations after 2235 Jacobian and 2861 function evaluations name coeff SE tstat pval gradient JSingval theta1 9330.89 NA NA NA 5.275e-11 967470 theta2 9330.41 NA NA NA -5.318e-11 1.772 theta3 -3.0032e-07 NA NA NA 1.389e-05 8.028e-12
Kind regards, Paul El dom, 20 ago 2023 a las 13:21, Bert Gunter (<bgunter.4...@gmail.com>) escribió: > 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.