30 May 2009

Semi-supervised or Semi-unsupervised? (SSL-NLP invited papers)

Kevin Duh not-so-recently asked me to write a "position piece" for the workshop he's co-organizing on semi-supervised learning in NLP. I not-so-recently agreed. And recently I actually wrote said position piece. You can also find a link off the workshop page. I hope people recognize that it's intentionally a bit tongue-in-cheek. If you want to discuss this stuff or related things in general, come to the panel at NAACL from 4:25 to 5:25 on 4 June at the workshop! You can read the paper for more information, but my basic point is that we can typically divide semi-supervised approached into one lump (semi-supervised) that work reasonably well with only labeled data and are just improved with unlabeled data and one lump (semi-unsupervised) that work reasonably well with only unlabeled data and are just improved with labeled data. The former are typically encode lots of prior information; the latter do not. Let's combine! (Okay, my claim is more nuanced than that, but that's the high-order bit.)

11 comments:

Rachel Cotterill said...

I was tempted to come to that workshop but I've signed up for the two semantics ones instead. Looks like I have a full week of semantics ahead, actually.

Rachel ~ writing from the Ballroom in UMC!

Bob Carpenter said...

I don't see the generative/discriminative distinction as such a dichotomy. One attractive feature of Bayesian inference is that once you've set up a model, arbitrary subsets of the random variables may be supervised, which amounts to simply fixing their outcomes.

With hierarchical Bayesian GLMs, one typically models (a) the hierarchical mean and variance parameters, (b) the regression coefficients, and (c) the predicted outcomes including noise. One cannot by definition model (d) the highest level priors. Typically, one does not model (e) the item level predictors.

But really this choice is arbitrary. You can choose not to model the hierarchical variance parameters or you can set the hiearchical prior means to zero, for instance. For missing data imputation, you see the predictors themselves being modeled to fill in missing data (often with an ad-hoc regression based on the other predictors).

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the predicted outcomes including noise. One cannot by definition model (d) the highest level priors. Typically, one does not model (e) the item level predictors.
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gamefan12 said...

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