[pymvpa] default SVM C parameter

J.A. Etzel jetzel at artsci.wustl.edu
Tue Sep 6 16:42:45 UTC 2011

Several years ago I played with using different values of c, by a 
rigorous grid-search and arbitrarily. I've never seen optimizing make 
enough of a difference with fMRI data to justify the increased hassle 
and time.

Anyone seen anything different?


On 9/1/2011 9:03 AM, Mark Lescroart wrote:
> Hello all,
> I have two questions about the default C parameter for linear SVM
> classifiers in pymvpa (/ LibSVM).
> First, I'd like a little more information about how the parameter is
> chosen by default. The help says that the default value is -1, and
> that "In linear kernel, negative values provide automatic scaling of
> their value according to the norm of the data," but I didn't find
> that particularly helpful (What does "according to the norm of the
> data" mean?)
> Second, does anyone have any experience choosing a C parameter by
> more rigorous methods (cross-validated parameter selection)? Does
> choosing an optimal C value make a big difference?
> Mark
> PS - I'm still using the 0.4 version of pymvpa - has setting of the
> default C parameter changed since then? I'm using the LibSVM
> implementation of the LinearCSVMC classifier on MacOS X 10.5.8.
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