[R] loess' robustness weights in loess
Prof Brian Ripley
ripley at stats.ox.ac.uk
Fri Apr 9 17:52:17 CEST 2004
On Fri, 9 Apr 2004, Rafael A. Irizarry wrote:
> i want to change the "robustness weights" used by loess. these
> are described on page 316 of chambers and hastie's "statistical models in S"
> book as
> r_i = B(e_i,6m)
> where B is tukey's biweight function, e_i are the residulas, and m is the
> median average distance from 0 of the residuals. i want to
> change 6m to, say, 3m.
> is there a way to do this? i cant figure it out from the help files.
Well, they say loess in R is an interface to C/Fortran code, and not the
same code as the S code described in Chambers & Hastie. I translated the
C driver routines to R for some added flexibility.
At a quick look, in function simpleLoess() you will find the weights in
object `robust', calculated by Fortran function lowesw. You could replace
that by a call to an R-level alternative, or play with the Fortran source
code. You'll have to do what I did way back, and read the source code to
see how it works in detail.
Brian D. Ripley, ripley at stats.ox.ac.uk
Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/
University of Oxford, Tel: +44 1865 272861 (self)
1 South Parks Road, +44 1865 272866 (PA)
Oxford OX1 3TG, UK Fax: +44 1865 272595
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