Machine Learning (Theory)

7/11/2011

Interesting Neural Network Papers at ICML 2011

Maybe it’s too early to call, but with four separate Neural Network sessions at this year’s ICML, it looks like Neural Networks are making a comeback. Here are my highlights of these sessions. In general, my feeling is that these papers both demystify deep learning and show its broader applicability.

The first observation I made is that the once disreputable “Neural” nomenclature is being used again in lieu of “deep learning”. Maybe it’s because Adam Coates et al. showed that single layer networks can work surprisingly well.

Another surprising result out of Andrew Ng’s group comes from Andrew Saxe et al. who show that certain convolutional pooling architectures can obtain close to state-of-the-art performance with random weights (that is, without actually learning).

Of course, in most cases we do want to train these models eventually. There were two interesting papers on the topic of training neural networks. In the first, Quoc Le et al. show that a simple, off-the-shelf L-BFGS optimizer is often preferable to stochastic gradient descent.

Secondly, Martens and Sutskever from Geoff Hinton’s group show how to train recurrent neural networks for sequence tasks that exhibit very long range dependencies:

It will be interesting to see whether this type of training will allow recurrent neural networks to outperform CRFs on some standard sequence tasks and data sets. It certainly seems possible since even with standard L-BFGS our recursive neural network (see previous post) can outperform CRF-type models on several challenging computer vision tasks such as semantic segmentation of scene images. This common vision task of labeling each pixel with an object class has not received much attention from the deep learning community.
Apart from the vision experiments, this paper further solidifies the trend that neural networks are being used more and more in natural language processing. In our case, the RNN-based model was used for structure prediction. Another neat example of this trend comes from Yann Dauphin et al. in Yoshua Bengio’s group. They present an interesting solution for learning with sparse bag-of-word representations.

Such sparse representations had previously been problematic for neural architectures.

In summary, these papers have helped us understand a bit better which “deep” or “neural” architectures work, why they work and how we should train them. Furthermore, the scope of problems that these architectures can handle has been widened to harder and more real-life problems.

Of the non-neural papers, these two papers stood out for me:

7/10/2011

ICML 2011 and the future

Unfortunately, I ended up sick for much of this ICML. I did manage to catch one interesting paper:

Richard Socher, Cliff Lin, Andrew Y. Ng, and Christopher D. Manning Parsing Natural Scenes and Natural Language with Recursive Neural Networks.

I invited Richard to share his list of interesting papers, so hopefully we’ll hear from him soon. In the meantime, Paul and Hal have posted some lists.

the future

Joelle and I are program chairs for ICML 2012 in Edinburgh, which I previously enjoyed visiting in 2005. This is a huge responsibility, that we hope to accomplish well. A part of this (perhaps the most fun part), is imagining how we can make ICML better. A key and critical constraint is choosing things that can be accomplished. So far we have:

  1. Colocation. The first thing we looked into was potential colocations. We quickly discovered that many other conferences precomitted their location. For the future, getting a colocation with ACL or SIGIR, seems to require more advanced planning. If that can be done, I believe there is substantial interest—I understand there was substantial interest in the joint symposium this year. What we did manage was achieving a colocation with COLT and there is an outside chance that a machine learning summer school will precede the main conference. The colocation with COLT is in both time and space, with COLT organized as (essentially) a separate track in a nearby building. We look forward to organizing a joint invited session or two with the COLT program chairs.
  2. Tutorials. We don’t have anything imaginative here, except for pushing for quality tutorials, probably through a mixture of invitations and a call. There is a small chance we’ll be able to organize a machine learning summer school as a prequel, which would be quite cool, but several things have to break right for this to occur.
  3. Conference. We are considering a few tinkerings with the conference format.
    1. Shifting a conference banquet to be during the workshops, more tightly integrating the workshops.
    2. Having 3 nights of posters (1 per day) rather than 2 nights. This provides more time/poster, and avoids halving talks and posters appear on different days.
    3. Having impromptu sessions in the evening. Two possibilities here are impromptu talks and perhaps a joint open problems session with COLT. I’ve made sure we have rooms available so others can organize other things.
    4. We may go for short presentations (+ a poster) for some papers, depending on how things work out schedulewise. My opinions on this are complex. ICML is traditionally multitrack with all papers having a 25 minute-ish presentation. As a mechanism for research, I believe this is superior to a single track conference of a similar size because:
      1. Typically some talk of potential interest can always be found by participants avoiding the boredom problem which comes up at a single track conference
      2. My experience is that program organizers have a limited ability to foresee which talks are of most interest, commonly creating a misallocation of attention.

      On the other hand, there are clearly limits to the number of tracks that are reasonable, and I feel like ICML (especially with COLT cotimed) is near the upper limit. There are also some papers which have a limited scope of interest, for which a shorter presentation is reasonable.

  4. Workshops. A big change here—we want to experiment with 2 days of workshops rather than 1. There seems to be demand for it, as the number of workshops historically is about 10, enough that it’s easy to imagine people commonly interested in 2 workshops. It’s also the case that NIPS has had to start rejecting a substantial fraction of workshop submissions for space reasons. I am personally a big believer in workshops as a mechanism for further research, so I hope this works out well.
  5. Journal integration. I tend to believe that we should be shifting to a journal format for ICML papers, as per many past discussions. After thinking about this the easiest way seems to be simply piggybacking on existing journals such as JMLR and MLJ by essentially declaring that people could submit there first, and if accepted, and not otherwise presented at a conference, present at ICML. This was considered too large a change, so it is not happening. Nevertheless, it is a possible tweak that I believe should be considered for the future. My best guess is that this would never displace the baseline conference review process, but it would help some papers that don’t naturally fit into a conference format while keeping quality high.
  6. Reviewing. Drawing on plentiful experience with what goes wrong, I think we can create the best reviewing system for conferences. We are still debating exact details here while working through what is possible in different conference systems. Nevertheless, some basic goals are:
    1. Double Blind [routine now] Two identical papers with different authors should have the same chance of success. In terms of reviewing quality, I think double blind makes little difference in the short term, but the public commitment to fair reviewing makes a real difference in the long term.
    2. Author Feedback [routine now] Author feedback makes a difference in only a small minority of decisions, but I believe its effect is larger as (a) reviewer quality improves and (b) reviewer understanding improves. Both of these are silent improvers of quality. Somewhat less routine, we are seeking a mechanism for authors to be able to provide feedback if additional reviews are requested, as I’ve become cautious of the late-breaking highly negative review.
    3. Paper Editing. Geoff Gordon tweaked AIStats this year to allow authors to revise papers during feedback. I think this is helpful, because it encourages authors to fix clarity issues immediately, rather than waiting longer. This helps with some things, but it is not a panacea—authors still have to convince reviewers their paper is worthwhile, and given the way people are first impressions are lasting impressions.
    4. Multisource reviewing. We want all of the initial reviews to be assigned by good yet different mechanisms. In the past, I’ve observed that the source of reviewer assignments can greatly bias the decision outcome, all the way from “accept with minor revisions” to “reject” in the case of a JMLR submission that I had. Our plan at the moment is that one review will be assigned by bidding, one by a primary area chair, and one by a secondary area chair.
    5. No single points of failure. When Bob Williamson and I were PC members for learning theory at NIPS, we each came to a decisions given reviews and then reconciled differences. This made a difference on about 5-10% of decisions, and (I believe) improved overall quality a bit. More generally, I’ve seen instances where an area chair has an unjustifiable dislike for a paper and kills it off, which this mechanism avoids.
    6. Speed. In general, I believe speed and good decision making are antagonistic. Nevertheless, we believe it is important to try to do the reviewing both quickly and well. Doing things quickly implies that we can push the submission deadline back later, providing authors more time to make quality papers. Key elements of doing things well fast are: good organization (that’s all on us), light loads for everyone involved (i.e. not too many papers), crowd sourcing (i.e. most decisions made by area chairs), and some amount of asynchrony. Altogether, we believe at the moment that two weeks can be shaved from our reviewing process.
  7. Website. Traditionally at ICML, every new local organizer was responsible for creating a website. This doesn’t make sense anymore, because substantial work is required there, which can and should be amortized across the years so that the website can evolve to do more for the community. We plant to create a permanent website, based around some combination of icml.cc and machinelearning.org. I think this just makes sense.
  8. Publishing. We are thinking about strongly encouraging authors to use arxiv for final submissions. This provides a lasting backing store for ICML papers, as well as a mechanism for revisions. The reality here is that some mistakes get into even final drafts, so a way to revise for the long term is helpful. We are also planning to videotape and make available all talks, although a decision between videolectures and Weyond has not yet been made.

Implementing all the changes above is ambitious, but I believe feasible and that each is individually beneficial and to some extent individually evaluatable. I’d like to hear any thoughts you have on this. It’s also not too late if you have further suggestions of your own.

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