Hi Dennis, Thank you for your suggestion. Firstly, the below minimum step size is only a warning because I told it to be (warnOnly=T), otherwise it's thrown as an error. I'm not sure if this is completely arbitrary. About segmented regression, I have tried to use it, but it doesn't seem to work right with weights, but I may have done something wrong, have you tried it with weights before?
Karen On Mon, Apr 19, 2010 at 3:59 PM, Dennis Murphy <djmu...@gmail.com> wrote: > Hi: > > On Mon, Apr 19, 2010 at 8:38 AM, Karen Chang Liu <kare...@uw.edu> wrote: > >> Hi, >> >> I have a small dataset that I'm fitting a segmented regression using nls >> on. >> I get a step below minimum factor error, which I presume is because >> residual >> sum of square is still "not small enough" when steps in the parameter >> space >> is already below specified/default value. However, when I look at the >> trace, >> the convergence seems to have been reached. I initially thought I might >> have >> reached the parameter space boundary, but these converging parameter >> values >> are by no means near boundary, they are quite in the middle. Could someone >> help me understand or throw out some possibilities? >> > > It's not an error; it's a warning. In this case, it's simply telling you > that the > step size it took was smaller than the minimum specified by your function > call. > If an error had been thrown, the function would have stopped and the object > > nlmod would not have been kept in your workspace. If the results 'look > right', > the warning is probably harmless; it's there to get your attention and warn > you > to look carefully at the results. (Good programming style, I'd say...) > > BTW, are you aware that there is a package named segmented on CRAN that > is reputedly designed for this type of problem? I mention this because the > nls > fit is not particularly good (the changepoint is too early and the slope is > too sharp...) > and perhaps this package can do a better job (hopefully). FYI... > > HTH, > Dennis > >> >> ##Here's a sample dataset and code. >> y2<-c(2.404529, 1.625661, 1.013981, 3.810921, 10.023745, 10.990817, >> 10.740636, 11.246827,17.022761, 21.430386) >> x2<-c(25.0, 29.3, 33.8, 38.3, 42.8, 47.2, 51.6, 55.8, 60.4, 64.9) >> dat <- data.frame(x2,y2) >> nlmod <- nls(y2 ~ ifelse(x2 < xint+(yint/slp), yint, yint + >> (x2-(xint+(yint/slp)))*slp), >> data=dat, control=list(minFactor=1e-5,maxiter=500,warnOnly=T), >> start=list(xint=40.49782, yint=1.013981, slp=0.8547828), >> na.action=na.omit, trace=T) >> >> ##plotting the function >> plot(dat$x2,dat$y2) >> segments(x0=0, x1=coef(nlmod)[1]+coef(nlmod)[2]*coef(nlmod)[3], >> y0=coef(nlmod)[2], y1=coef(nlmod)[2]) >> segments(x0=coef(nlmod)[1]+coef(nlmod)[2]*coef(nlmod)[3],x1=80, >> y0=coef(nlmod)[2], y1=80*coef(nlmod)[3]+coef(nlmod)[2]) >> >> Karen >> >> [[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. >> > > -- Karen Liu Graduate Student Department of Biostatistics, University of Washington Seattle, WA 98105 kare...@uw.edu [[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.
