[pymvpa] How to evaluate the goodness of classification for an unlabelled example?

Yaroslav Halchenko debian at onerussian.com
Fri Jan 18 14:23:40 UTC 2013

On Fri, 18 Jan 2013, Roberto Guidotti wrote:

>    Dear all,
>    I have a question that do not strictly concern to PyMVPA strictly.
>    I trained a classifier to discriminate two classes (e.g. bananas and
>    apples), using SVM, cross-validation etc. then I would like to try it with
>    some "unlabelled" fruits, could be, bananas and apples but also melon,
>    lemon, strawberries. If I try to classify a melon, the label assigned by
>    the classifier could be banana. How can I establish a probability level
>    for this fruit? I mean, if I use SVM distance from the hyperplane, the
>    melon could be distant from bananas and further from apples (hyperspaces)
>    and thus in my opinion this is not a good index for that. I would like to
>    have an index that tries to tell me that is a banana only with higher
>    probability than apples: p(bananas) = 0.3 p(apple) = 0.1 for example.

What about using SMLR -- as a logistic regression its decision is based
on the max of probabilities per each possible (trained) label.  So just
enable_ca=['estimates'] and there (in .ca.estimates) you would get your
probabilities per each target label for the last .predict call

if for SVM - enable estimation of probabilities (I believe a sigmoid is
fit by libsvm in the decision boundary neighborhood) " probability=1"
and then get them from .ca.probabilities

or some other classifier? GDA/LDA/GNB...

would that help?
>    Hope it is an xhaustive and an answerable question!�
>    Thank you
>    Roberto

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Yaroslav O. Halchenko
Postdoctoral Fellow,   Department of Psychological and Brain Sciences
Dartmouth College, 419 Moore Hall, Hinman Box 6207, Hanover, NH 03755
Phone: +1 (603) 646-9834                       Fax: +1 (603) 646-1419
WWW:   http://www.linkedin.com/in/yarik        

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