[R] Model checking for gam (mgcv) result

[Simon Wood]

The curve is caused by the zeroes in your data.

raw.residual = response - fitted

so if response=0 then

raw.residual = -fitted.

You get a curve, rather than a straight line, on the fitted vs residual 
plot because the residuals are standardised in a way that also depends 
on the fitted value. (This happens with other GLMs too, not just GAMs)

best,
Simon

On 12/04/11 16:02, Samuel Turgeon wrote:
> Dear list,
>
> i'm checking the residuals plots of a gam model after a processus of model
> selection. I found the "best" model, all my terms are significant, the
> r-square and the deviance explained are good, but I have strange residuals
> plots:
>
> http://dl.dropbox.com/u/1169100/gam.check.png
> http://dl.dropbox.com/u/1169100/residuals_vs_fitted.png
>
> What does explains the "curve" on those plots??
>
> I've also plotted each explanatory variables included in the model against
> residuals and everything looks fine.
>
> Kind regards,
>
> Sam
>
> 	[[alternative HTML version deleted]]
>
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-- 
Simon Wood, Mathematical Science, University of Bath BA2 7AY UK
+44 (0)1225 386603               http://people.bath.ac.uk/sw283