[R] setting weights for such a two-class problem in nnet and svm

Prof Brian Ripley ripley at stats.ox.ac.uk
Fri Jul 22 18:56:12 CEST 2005

On Fri, 22 Jul 2005, Baoqiang Cao wrote:

> Dear All,
> I have such a two-class problem, one class is very large(~98% of total), 
> and the other is just 2%. According to manual of nnet, I need setup 
> "weights", so I intend to set 1 for class one, 49 for class 2. How do I 
> do that? Just weights=49?

I do not know what you mean by `manual of nnet', but no, not that since 
the help page says

   weights: (case) weights for each example - if missing defaults to 1.

More likely you want ifelse(class==1, 1/49, 1).

However, there is more to it than that, and do please read the reference 
books on the help page.

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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