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.berton@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.amatulli@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]]
> >
> > ______________________________________________
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> > 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

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