I agree. I typically do not use non-linear functions, so am seeing the "art" in describing functions of non-linear plots. One last thing. I tried to use a self-starting Weibull function with the posted data and received the following error.
model<-nls(Level~ SSweibull(Time,Asym,Drop,lrc,pwr)) Error in qr.default(.swts * attr(rhs, "gradient")) : NA/NaN/Inf in foreign function call (arg 1) I do not understand the error statement. kc On Thu, Aug 26, 2010 at 1:44 PM, Bert Gunter <gunter.ber...@gene.com> wrote: > My opinions only below; consume at your own risk. > > On Thu, Aug 26, 2010 at 2:20 PM, Marlin Keith Cox <marlink...@gmail.com> > wrote: > > The background you requested are energetic level (joules) in a group of > > starved fish over a time period of 45 days. Weekly, fish (n=5) were > removed > > killed and measured for energy. This was done at three temperatures. I > am > > comparing the rates at which the fish consume stored body energy at each > of > > the three temperatures. Initial data looks like the colder fish > > have different rates (as would be expected) than do warmer fish. In all > > cases the slope is greatest at the beginning of the curve and flattens > after > > several weeks. This is what is interesting - where in time the line > > starts to flatten out. > > > > By calculating a non-linear equation of a line, I was hoping to use the > > first and second derivatives of the function to compare and explain > > differences between the three temperature. > > Bad idea. Derivatives from fitted curves are generally pretty > imprecisely determined. And you don't need them: If the curves are > being (adequately/appropriately) parameterized as Weibull, then all > the information is in the parameters anyway, which can be directly > modeled, fitted, and compared as functions of temperature -- provided > that the design permits this (i.e. provides sufficient precision for > the characterizations/comparisons). > > If you don't know how to do this, seek further statistical help. > > -- > Bert Gunter > Genentech Nonclinical Statistics > > > > > > The data originally posted was an example of one of the curves > experienced. > > > > kc > > > > On Thu, Aug 26, 2010 at 9:48 AM, David Winsemius <dwinsem...@comcast.net > >wrote: > > > >> > >> On Aug 26, 2010, at 1:35 PM, Marlin Keith Cox wrote: > >> > >> I need the parameters estimated for a non-linear equation, an example > of > >>> the > >>> data is below. > >>> > >>> > >>> # rm(list=ls()) I really wish people would add comments to > destructive > >>> pieces of code. > >>> > >> > >> Time<-c( 0, 0, 0, 0, 0, 2, 2, 2, 2, 2, 3, 3, 3, 3, 3, 4, 4, 4, > >>> 4, 4, 5, 5, 5, 5, 5, 8, 8, 8, 8, 8) > >>> Level<-c( 100, 110, 90, 95, 87, 60, 65, 61, 55, 57, 40, 41, > 50, > >>> 47, > >>> 44, 44, 42, 38, 40, 37, 37, 35, 40, 34, 32, 20, 22, 25, > 27, > >>> 29) > >>> plot(Time,Level,pch=16) > >>> > >> > >> You did not say what sort of "non-linear equation" would best suit, nor > did > >> you offer any background regarding the domain of study. There must be > many > >> ways to do this. After looking at the data, a first pass looks like > this: > >> > >> > lm(log(Level) ~Time ) > >> > >> Call: > >> lm(formula = log(Level) ~ Time) > >> > >> Coefficients: > >> (Intercept) Time > >> 4.4294 -0.1673 > >> > >> > exp(4.4294) > >> [1] 83.88107 > >> > points(unique(Time), exp(4.4294 -unique(Time)*0.1673), col="red", > pch=4) > >> > >> Maybe a Weibull model would be more appropriate. > >> > >> > >> -- > >> > >> David Winsemius, MD > >> West Hartford, CT > >> > >> > > > > > > > [[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<http://www.r-project.org/posting-guide.html> > > and provide commented, minimal, self-contained, reproducible code. > > > > > > -- > Bert Gunter > Genentech Nonclinical Biostatistics > 467-7374 > http://devo.gene.com/groups/devo/depts/ncb/home.shtml > -- M. Keith Cox, Ph.D. Alaska NOAA Fisheries, National Marine Fisheries Service Auke Bay Laboratories 17109 Pt. Lena Loop Rd. Juneau, AK 99801 keith....@noaa.gov marlink...@gmail.com U.S. (907) 789-6603 [[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.
