R help - Mar 2009 - Difference between gam() and loess().

It seems that in general

	gam(y~lo(x)) # gam() from the gam package.

and
	loess(y~x)

give slightly different results (in respect of the predicted/fitted  
values).
Most noticeable at the endpoints of the range of x.

Can anyone enlighten me about the reason for this difference?

Is it possible to twiddle the control parameters, for either or both  
functions,
so as to obtain identical results?

Thanks.

	cheers,

		Rolf Turner

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Rolf Turner wrote:
> 
> It seems that in general
> 
>     gam(y~lo(x)) # gam() from the gam package.
> 
> and
>     loess(y~x)
> 
> give slightly different results (in respect of the predicted/fitted 
> values).
> Most noticeable at the endpoints of the range of x.
> 
> Can anyone enlighten me about the reason for this difference?
> 
> Is it possible to twiddle the control parameters, for either or both 
> functions,
> so as to obtain identical results?
There are two obvious differences in the defaults.  In lo() from the gam 
package, span=0.5 and degree=1 while for loess(), span=0.75 and degree=2.

Try gam(y~lo(x,span=0.75,degree=2)) and see if that helps.

Kevin

-- 
Kevin E. Thorpe
Biostatistician/Trialist, Knowledge Translation Program
Assistant Professor, Dalla Lana School of Public Health
University of Toronto
email: kevin.thorpe at utoronto.ca  Tel: 416.864.5776  Fax: 416.864.6057