You appear to be quite confused. I have no idea what your "a" and "b" below mean. The SAS documentation that you quote is for prediction from a bivariate normal. I don't know what that has to do with your problem, nor with car's confidence ellipses for parameters for a univariate regression. I would suggest you get local statistical help, though perhaps someone with more time on their hands on this list than I may help sort you out.
Cheers, Bert On Sun, Jan 27, 2013 at 8:41 AM, Giuseppe Amatulli <giuseppe.amatu...@gmail.com> wrote: > Hi, > thanks for your replay. > My values of a and b are respectively: > a = observation of an event > b = prediction of a model. > > Therefore i would like to draw the confidence region for predicting a new > observation, and according to this > http://v8doc.sas.com/sashtml/insight/chap40/sect35.htm the prediction > ellipse should be more appropriate. > > But i'm not able to track back the function > radius <- sqrt(dfn * qf(level, dfn, dfd)) > in order to change it and draw a prediction ellipses. > > Regards > Giuseppe > > > > > On 26 January 2013 17:19, Bert Gunter <gunter.ber...@gene.com> wrote: >> >> Well, I'd guess you have to first define what you mean by "prediction >> ellipse," as the confidence ellipses are for the bivariate >> distribution of 2 parameter estimates -- as I understand it -- >> whereas predictions depend on the covariate values and are for a >> single response value (unless you have fitted multiple responses, I >> suppose). >> >> -- Bert >> >> On Sat, Jan 26, 2013 at 1:12 PM, Giuseppe Amatulli >> <giuseppe.amatu...@gmail.com> wrote: >> > Hi, >> > I'm using the R library(car) to draw confidence/prediction ellipses in a >> > scatterplot. >> > >From what i understood the ellipse() function return an ellipse based >> > parameters: shape, center, radius . >> > If i read dataEllipse() function i can see how these parameters are >> > calculated for a confidence ellipse. >> > >> > ibrary(car) >> > >> > a=c(12,12,4,5,63,63,23) >> > b=c(13,15,7,10,73,83,43) >> > >> > v <- cov.trob(cbind(a, b)) >> > shape <- v$cov >> > center <- v$center >> > >> > radius <- sqrt(2 * qf(0.95, 2, length(a) - 1)) # radius <- sqrt(dfn * >> > qf(level, dfn, dfd)) >> > >> > conf.elip = ellipse(center, shape, radius,draw = F) >> > plot(conf.elip, type='l') >> > points(a,b) >> > >> > My question is how I can calculate shape, center and radius to obtain a >> > prediction ellipses rather than a confidence ellipse? >> > Thanks in Advance >> > Giuseppe >> > >> > -- >> > Giuseppe Amatulli >> > Web: www.spatial-ecology.net >> > >> > [[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. >> >> >> >> -- >> >> Bert Gunter >> Genentech Nonclinical Biostatistics >> >> Internal Contact Info: >> Phone: 467-7374 >> Website: >> >> http://pharmadevelopment.roche.com/index/pdb/pdb-functional-groups/pdb-biostatistics/pdb-ncb-home.htm > > > > > -- > Giuseppe Amatulli > Web: www.spatial-ecology.net -- Bert Gunter Genentech Nonclinical Biostatistics Internal Contact Info: Phone: 467-7374 Website: http://pharmadevelopment.roche.com/index/pdb/pdb-functional-groups/pdb-biostatistics/pdb-ncb-home.htm ______________________________________________ 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.
