R help - Jul 2005 - setting weights for such a two-class problem in nnet and svm

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?
Meanwhile I'd like to try svm(e1071), again, how do I setup
"class.weights"? Thanks.

BTW: Many thanks to Jake and Uwe for their answers about using "save"
to store trained networks!


Best regards,     
 Baoqiang Cao

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

Dear Baoqiang,

there is an example on the svm Help page on the use of 'class.weights'. 

HTH
David


----------------

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? 
Meanwhile I'd like to try svm(e1071), again, how do I setup
"class.weights"? Thanks.

-- 
Dr. David Meyer
Department of Information Systems and Operations

Vienna University of Economics and Business Administration
Augasse 2-6, A-1090 Wien, Austria, Europe
Fax: +43-1-313 36x746 
Tel: +43-1-313 36x4393
HP:  http://wi.wu-wien.ac.at/~meyer/