Roger, thank you for looking into this. However, the posting guide asked the poster to contact the maintainer. Had (s)he done so, I would have pointed out that spatial 7.28-2 (the current version for R-devel) has this corrected (in a slightly simpler way).
On Thu, 10 Aug 2006, Roger Bivand wrote: > Yan Wong <h.y.wong <at> leeds.ac.uk> writes: > > > > > Although the 'spatial' documentation doesn't mention that extractAIC > > works, it does seem to give an output. > > Could I suggest moving this question to the R-sig-geo list? > > Please note that surf.ls() converts x and y to the [-1, +1] range to ensure > that > higher powers of possibly very large absolute coordinate values do not cause > trouble, so that the surf.ls() and lm() models may differ anyway. > > I believe that there is a bug in extractAIC.trls() - which I contributed to > the > spatial package some years ago, with edf <- df.residual.trls(fit) rather than > n > - df.residual.trls(fit). When this is corrected, for this case, the > extractAIC() > results agree. > > Roger Bivand > > > I may have misunderstood, but shouldn't the following give at least > > the same d.f.? > > > > > library(spatial) > > > data(topo, package="MASS") > > > extractAIC(surf.ls(2, topo)) > > [1] 46.0000 437.5059 > > > extractAIC(lm(z ~ x+I(x^2)+y+I(y^2)+x:y, topo)) > > [1] 6.0000 357.5059 > > > > # and if the AIC values differ, shouldn't they do so by an additive > > constant? > > > > > (extractAIC(surf.ls(2, topo))-extractAIC(lm(z ~ x+I(x^2)+y+I(y^2) > > +x:y, topo)))[2] > > [1] 80 > > > (extractAIC(surf.ls(1, topo))-extractAIC(lm(z ~ x+y, topo)))[2] > > [1] 92 > > > > # Using R 2.3.1 (OS X), spatial version 7.2-27.1 > > > > Thanks > > > > Yan -- Brian D. Ripley, [EMAIL PROTECTED] Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/ University of Oxford, Tel: +44 1865 272861 (self) 1 South Parks Road, +44 1865 272866 (PA) Oxford OX1 3TG, UK Fax: +44 1865 272595 ______________________________________________ [email protected] 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.
