Try to set your own starting values. Use the values that you would expect for S0, mu and sigma.
HTH, Thierry ---------------------------------------------------------------------------- ir. Thierry Onkelinx Instituut voor natuur- en bosonderzoek / Research Institute for Nature and Forest Cel biometrie, methodologie en kwaliteitszorg / Section biometrics, methodology and quality assurance Gaverstraat 4 9500 Geraardsbergen Belgium tel. + 32 54/436 185 [EMAIL PROTECTED] www.inbo.be To call in the statistician after the experiment is done may be no more than asking him to perform a post-mortem examination: he may be able to say what the experiment died of. ~ Sir Ronald Aylmer Fisher The plural of anecdote is not data. ~ Roger Brinner The combination of some data and an aching desire for an answer does not ensure that a reasonable answer can be extracted from a given body of data. ~ John Tukey -----Oorspronkelijk bericht----- Van: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] Namens Gunther Höning Verzonden: dinsdag 1 juli 2008 14:41 Aan: [email protected] Onderwerp: [R] Regression and fitting Dear list, I have some values like Time 2 4 8 24 48 72 UTR 82543 169105 207615 96633 31988 7005 UTRs 82687 172934 205541 101842 31898 6950 of a twice repeated meassurement. I know that the underlying function is of : f(x) = SO/(sqrt(2*pi)*sigma *x) * exp(-(S0*ln x - mu)^2/(2*sigma^2)). How can I determine the value of S0, sigma and mu. I tried nls: lognormal <- function(x,S0,sigma,mu) {S0/(sqrt(2*pi)*sigma *x) * exp(-(S0*log (x) - mu)^2/(2*sigma^2))} x <- rep(c(2,4,6,24,48,72),2) y <- c(82543,169105,207615,96633,31988,7005,82687,172934,205541,101842,31898,6950 ) dat <- data.frame(x,y) regfit <- nls(y ~ lognormal(x,S0,sigma,mu) ,data =dat) Output: Error in nlsModel(formula, mf, start, wts) : singular gradient matrix at initial parameter estimates Warning message: No starting values specified for some parameters. Intializing 'S0', 'sigma', 'mu' to '1.'. Consider specifying 'start' or using a selfStart model in: nls(y ~ lognormal(x, S0, sigma, mu), data = dat) Can anybody help on this topic please ? Gunther ______________________________________________ [email protected] 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. ______________________________________________ [email protected] 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.
