[R] extractAIC using surf.ls
Roger Bivand
Roger.Bivand at nhh.no
Thu Aug 10 09:49:34 CEST 2006
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
>
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