Message-ID: <49C2E146.5000702@utoronto.ca>
Date: 2009-03-20T00:20:22Z
From: Kevin E. Thorpe
Subject: Difference between gam() and loess().
In-Reply-To: <565B0378-4D10-4904-9AC7-3F8896890C07@auckland.ac.nz>
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