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 [[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.
