[SciPy-user] Academic Question?

Aarre Laakso aarre at pair.com
Mon Aug 28 16:55:22 CDT 2006

Conor Robinson wrote:
> If one is using 1ofC encoding, what does PCA
> applied to 1ofC result in?  1ofc is nice if you're trying to get the
> posterior probability distribution as output (sigmoid single output
> unit feedforward network),
> however would this still hold true after applying PCA to reduce input
> dimensionality?  Does this even make "sense" for 1ofc?  Furthermore,
> is there any solid literature reviewing encoding schemes for nnets
> etc?

>From what I've been told, it doesn't make sense to apply PCA to
categorical variables, although it is a common practice.

If you want the posterior probability distribution at the outputs, then
I believe you want to use softmax.

The neural nets FAQ http://www.faqs.org/faqs/ai-faq/neural-nets/ has
some practical advice about encoding, with citations to the literature
(part 2, "How should categories be encoded?"), as well as some
recommendations on the literature in general (part 4). If you haven't
read Bishop (1995), I highly recommend it.


Aarre Laakso

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