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
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Simon Wood, Mathematical Science, University of Bath BA2 7AY UK
+44 (0)1225 386603 http://people.bath.ac.uk/sw283
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