ALT 2017

I’ve just returned from the 28th International Conference on Algorithmic Learning Theory (ALT 2017), which was held in Kyoto, Japan.  The last time ALT was held in Japan was in 2007, exactly 10 years ago.  Back then, I was a second year grad student at my first ALT, and at one of my first conferences altogether.  Now, ten years later, I got to watch my Ph.D. student Mano Vikash present his first conference paper, and I was serving as program co-chair, together with Steve Hanneke.  What a difference 10 years makes!

a photo I took at Yasaka Shrine in Kyoto

Except for handling some last-minute duties concerning session chair assignments and minor issues with the online proceedings, there was little left for me and Steve to do at the actual conference. Once the conference started, the local organizers and general chair took over, and they kept things running smoothly.  But the experience of attending was still different for me.  By the time the conference started, I had read many of the papers and was at least a little familiar with the rest of them.  I also felt a strange sense of responsibility to go to every single talk, and I enjoyed doing this more than I expected.

Being PC chair was a lot of work, but it was quite gratifying.  I appreciate that Steve and I were able to work quite well together.  I’m thankful that so many amazing people agreed to serve on the PC and that our invited speakers readily agreed to come all the way to Japan.  I am thankful that we got many good submissions — more submitted papers than in any year since my first ALT a decade ago.  And I am especially grateful for the quality of the reviews; the PC had to handle more papers than usual, yet the reviews were careful and detailed, catching multiple bugs and providing valuable feedback to authors.  The resulting proceedings are online.

We also had two great invited speakers for ALT.  Sasha Raklin, among other things, gave some interesting results that I was not previously aware of at the intersection of Rademacher complexity and online learning.  Adam Kalai gave great and accessible talk on "fairness" and had a nice way to express various notions of fairness as loss functions. I must admit I’ve been skeptical of this area for a while, but chatting with Adam afterwards has made me less so.  

When blogging about conferences in the past, I discussed some of my favorite papers.  I don’t feel it would be especially appropriate for me to do so in this case, so I’ll just mention the paper for which the E. M. Gold student paper award was given to Tyler Dorhn, an undergraduate student at Yale.  In the paper, Dana Angluin and Tyler Dohrn showed that when equivalence queries are answered randomly, the expected query complexity of exact learning is drastically improved over when equivalence queries are answered adversarially.  In doing so, they introduced a nice notion called the "elimination graph" of a concept space.  It's a concept that I expect to have more applications.   And this is a problem I and others have informally thought about, so I’m glad to see progress in this area.

Finally, I’ll note that ALT has been going through some changes lately.  This year, in addition to more minor tweaks, we switched publication venues from Springer to PMLR (the new name for JMLR’s conference proceedings) in favor of open access, and we got rid of page limits.  More big changes are coming next year: ALT 2018 also will co-locate with AISTATS instead of DS next year, and PC co-chairs Mehryar Mohri and Karthik Sridharan have put out an ambitious call for papers with the goal of becoming the "best conference in algorithmic and theoretical machine learning."  (The co-location with AISTATS also means that the conference is moving from Fall to Spring, and papers are due in a week!)

Computational learning theory has two main conferences: COLT and ALT, with COLT being the larger of the two.  ALT has always had strong authors and PC members, but hadn’t grown in prestige and visibility like COLT.  My former postdoc host John Langford wrote, "ALT = 0.5 COLT." Yet, I’ve always appreciated ALT’s breadth and its resilience to various trends that change the theme of some other conferences almost yearly.  ALT grew this year, and I’m optimistic about its future. And my hope is that ALT can meet its new ambitions while retaining its friendly and open culture.

On AAAI 2014

I've just returned from AAAI 2014, which was held in Quebec City, Canada. It was my first time submitting to, attending, or presenting at a AAAI, and I was very pleasantly surprised on many levels -- the strength of the program, the breadth of research topics covered, and the conference organization.  I also really enjoyed spending some time in Quebec City, which was a wonderful location.

a photo of Quebec City that I took from the conference hotel

Organization

AAAI is quite a large conference with around a thousand attendees and a couple hundred presenters, so it involves multiple parallel tracks, posters, etc.  Carla Brodley and Peter Stone, the co-chairs, did a wonderful job organizing everything to work smoothly.  Moreover, they introduced two innovations that I particularly liked: 

1. Authors whose papers were selected for talks were able to choose to give a 2-minute plenary talk instead of a regular session talk.  This worked really well for papers that carried a strong conceptual message, which might appeal to a wider audience. (I chose this option: you can see my slides here.)  I was never bored at the plenary sessions -- if I found one of these talks or topics uninteresting, it was over in two minutes.  On the other hand, if I liked a talk, I could find the presenter later at the poster session (see next innovation).  The slides for these talks were all pre-loaded on one machine, and Carla and Peter somehow got everyone to stick to just two minutes, so, much to my surprise, these sessions went very smoothly.
2. Authors who didn't give talks and authors who chose the 2-minute plenary talk option had scheduled 45 minute slots to "present" their posters during one of the multiple poster sessions.  At any given time during a poster session only about 30 posters were officially being presented (you could also show people your poster and work at any time). This was great because I didn't feel like I had to stand by my poster for hours.  Moreover, during my 45 minute presentation, I had a much larger audience.  I really liked both innovations and hope they are incorporated into future AAAIs (and ICMLs, NIPSs, etc.). 

I also want to discuss some of the content that I enjoyed.  I should note that I left the conference day early, attended a somewhat arbitrary selection of sessions, and pseudorandomly meandered through the poster sessions, so what follows is not nearly complete.  I know for sure that I am leaving out many good talks and papers.

Plenary Talks

I only saw two of the invited talks, but both were really great.

image from mashable

In the first talk, Adam Cheyer, one of the main inventors of the iPhone "digital assistant" Siri, discussed the history of the project, as well as some of the technology behind it.  One thing I learned from the talk was how much Siri is able to do that I didn't know about.  For instance, if you're reading an email and want to call the email's sender, all you need to do is say something like "Siri, call him" if the contact info is in your phone.  If you think about it, this is incredibly easier than having to leave email, go to phone contacts, search, etc.  In fact, Adam pointed out that while your hands are the best interface for getting at items on a screen, your voice is better for accessing content you can't see.  If one uses Siri with this in mind, it can be much more effective.  Adam also pointed out how immensely challenging building the system was.  For instance, a user who wants two tickets for the movie "Nine" at "8pm" can say: "two for nine at eight" and Siri has to figure out what to do after hearing what seems like 4 different numbers.  Just thinking about the difficulty of this problem should impress us how for AI has come!

As for Siri's history, the entire recounting was interesting, but one facet stuck out the most:  Immediately after the iPhone was announced in 2007, Adam and two of his colleagues met to discuss how to take advantage of the new technology.  Just three months later, they were making a pitch to VCs to fund their new idea.  By 2010 Siri was an app, and by 2011 it was bought out and integrated into iOS, making billions of dollars for Apple and impacting millions of users.  Adam clearly had a lot of experience working on these types of systems and in launching new and exciting products (CALO, IRIS, etc.), but it amazed me how much one can accomplish in a short time if one is talented, experienced, and motivated enough.

an example UI from the coloring game described below, taken from this paper

The second invited talk was by Michael Kearns, who is known for a lot of things including fundamental contributions to learning theory.  But in this talk, he discussed some of the user studies in network science research he and his group at UPenn have been doing.  In these experiments, subjects come to a room and sit at computers and perform various tasks like distributedly coloring a graph, coming to consensus, etc.  In their typical setup, players play on an underlying graph (whose topology they don't know), with players controlling one vertex each and only seeing what their neighbors are doing.  

In a coloring game, the players have to choose one of a few colors and will only get paid if their final color is different from the colors chosen by all their neighbors.  They can asynchronously switch their own color as much as they want, and the goal is to see how long it takes various graphs to get colored.  In a consensus game, the goal is for the entire graph to come to consensus, so the players need to choose colors that are the same as their neighbors' choices.  From the computational complexity standpoint, coloring is NP-hard (in the worst case) and consensus is trivial, but Kearns's group observed that the hardness of performing these tasks in the real world depends on the interplay of the graph structure and the task.  For instance, on many graphs, consensus appears much harder than coloring.

The result I found the most interesting pertained to an experiment they ran on coming to consensus in a game where different nodes have different preferences.  The way this setting works is that the players only get paid if at the end of a set amount of time, all have agreed on a single color.  However, different players will be paid different amounts depending on which color they agree on.  (This was made to be a simplified model the Obama vs. Hillary primary, where democrats had strong reasons to come to agreement, but had different preferences about whom to agree on.)  Kearns's group showed that in such a game, a small minority of well connected players with one set of preferences in all their trials override a much larger set of worse connected players with different preferences, even though the minority in question had no information about the structure of the graph (or even any knowledge that they were more connected than others).

This seems an interesting line of research and could hope to explain various real-world phenomena.

Papers

Finally, I also enjoyed quite a few of the papers at the conference.  One thing that surprised me was the large number of machine learning and also game theory papers.  The learning / game theory papers were on average more application driven than what one usually finds at ICML/NIPS or EC, but it seems that these fields are becoming increasingly incorporated into AAAI.

So, here's a very incomplete list of some papers I found interesting:

• Active Learning with Model Selection by Alnur Ali, Rich Caruana, and Ashish Kapoor -- this paper considers active learning for  simultaneously choosing the best model and its parameters.  This is done by training the model on a training set and evaluating its performance on a validation set.  Both sets require labeled data.  So the active learning algorithm, in sampling a label for a new point, must also consider whether to add the point into the training set or into the unbiased validation set.  The resulting performance of their algorithm shows that it roughly first "explores" by placing into valuation and then "explores" by placing into training.  I found this idea and interpretation pretty neat.
• Fixing a Balanced Knockout Tournament by Haris Aziz, Serge Gaspers, Simon Mackenzie, Nicholas Mattei, Paul Stursberg, and Toby Walsh -- this paper considers a natural, previously open, question.  Given you know the probabilities with which each team will beat each other team, can you design a knockout tournament (balanced binary tree) that maximizes the probability of a given winning?  This turns out to be NP-hard, but they also give some positive results in certain cases.
• Preference Elicitation and Interview Minimization in Stable Matchings by Joanna Drummond and Craig Boutilier -- consider medical residents being matched to schools.  For residents and schools to choose their rankings, they need to interview with all the schools.  This is really costly.  This paper assumes that when one of two options is much better than the other, an interview probably doesn't need to be scheduled just to tell them apart.  Then, given their modeling assumptions, they try to find stable matchings without having to do too many expensive "interviews."  What I like about this paper is that it considers the sort of practical concerns that give a new twist to what seems like an already solved problem.
• Simultaneous Cake Cutting by Eric Balkanski, Simina Brânzei, David Kurokawa, and Ariel D. Procaccia -- this paper looks at "cake cutting" in the simultaneous model, where user preferences are elicited simultaneously.  Cake cutting problems involve dividing a resource, say a yummy continuous cake, such that everyone is happy with the division (or at least thinks the division fair under various notions).  They show that in a simultaneous model, one can satisfy "proportionality" but not "envy-freeness", and also give some approximation results.  I don't know exactly why I like this model, but I have a soft spot for cake cutting.
• Bagging by Design (on the Suboptimality of Bagging) by Periklis Papakonstantinou, Jia Xu, and Zhu Cao -- this paper gives a subsampling method that outperforms bagging, in the case, roughly, when bagging works, e.g. by being more stable to noise than a single predictor.  Their method comes from the observation that bagging is done over subsampling a fixed dataset, which makes bagging behave differently than if it had access to an unlimited supply of independent samples.
• Recovering from Selection Bias in Causal and Statistical Inference by Elias Bareinboim, Jin Tian, and Judea Pearl -- this paper won the best paper award.  It gives complete conditions on when one can recover from selection biased data. They also consider settings where some unbiased data is available and other extensions.  I plan on reading it more carefully, as the results seem important.

In sum, I enjoyed AAAI and look forward to attending it again in future years.

ALT 2011

Earlier this month, I traveled to Finland, where I attended ALT 2011 and presented a paper on learning sparse parity functions in the presence of noise. 

a picture I took in Helsinki

COLT (Conference on Learning Theory) and ALT (Algorithmic Learning Theory) are the two main learning theory conferences.  Though COLT is both the larger and better known of the two, it seems that ALT is growing stronger with every passing year, and this year was no exception.  Not only was the program good, but Sébastien Bubeck gave a timely tutorial on bandits, which deservedly got their own session this year. (I, unfortunately, arrived too late to see the tutorial, but I read the nice slides afterwards.)

I wouldn't be the first to note that the other thing that differentiates ALT from its big brother is that ALT seems to have a broader definition of learning theory, and the conference is therefore more diverse in its topics than a typical COLT.  This has clear benefits, but it also means that there are entire sessions where I am pretty lost.  Of course, as I've previously noted, not feeling like you have to following everything could also be a benefit when you're attending a conference.

I want to mention one paper I particularly enjoyed, especially because of the model, which illustrates the importance of unlabeled data in the agnostic setting:

"Learning a Classifier when the Labeling is Known" by Shai Ben-David and Shalev Ben-David.  This paper works in an agnostic learning model where the learner already knows the labeling function, but wants to find a good succinct representation (i.e. proper learning).  How good a particular representation is depends on the distribution the data is coming from, and this is what the algorithm needs to learn by seeing examples.  It turns out that this, more generous, model is subject to essentially the same sample complexity lower bound as the common agnostic PAC model.

On ICML 2011

I've heard from some people that they liked my summary of NIPS 2010, so I'm posting my thoughts on ICML 2011, where I went a couple weeks ago.  ICML is one of the two main yearly conferences in machine learning, so there are always interesting new results to see, and this year saw a record of over 700 attendees.  Unfortunately, I didn't get to attend the tutorials or the workshops, so my post is about the main conference.  The conference took place in Bellevue, a city near Seattle, Washington.

Bellevue downtown at night, from Wikipedia

Invited Talks

One of the main highlights of ICML 2011 were the invited talks.  Usually, I like one invited talk or so, but this time all of them were great.   I've heard a number of people remark that the organizers did a great job of choosing the speakers.  I should also note that the videos of all the talks, invited and not, will soon appear on TechTalks, and I recommend you watch the invited talks especially.

The first talk was by Chris Bishop, of the Microsoft Research (MSR) Cambridge, UK lab, who talked about applied machine learning coming of age.  He addressed a variety of topics, perhaps a bit loosely tied together, but the real highlight of his talk was the first half-hour, where he addressed the technology behind Kinect.  Apparently, much of the technology behind it was developed at MSR (this should be good for research funding at Microsoft, as Kinect was a huge commercial success), and he went into some depth about both the machine learning and hardware challenges in building Kinect. For example, machine learning was used for the body tracking system in Kinect, but with some interesting twists, e.g. 1) the body tracking problem isn't treated as a tracking problem at all: each individual frame is independently classified, 2) lots of the training data is synthetic.  On the hardware end, I learned that Kinect could track you just fine in the dark.  To find out more, you should watch the entire talk.

Kinect sensor, image from Wikipedia

The second invited talk was by a mathematical biologist, Martin Nowak, who talked about the evolutionary dynamics of competition and cooperation.  He talked about all sorts of interesting things.  What stuck with me the most were some results he presented on the repeated prisoner's dilemma.  In the famous classic prisoner's dilemma, each of two prisoners must independently decide whether to cooperate or defect (see figure below for a possible payoff matrix).  The dominant strategy for both prisoners is to defect, but if both prisoners cooperate they are better off than both defecting, hence the dilemma.  In the real world, we often play variants of this game, but over and over again.  The repeated prisoners dilemma setting captures this aspect of repeated play, and a while ago, Robert Axelrod held a famous repeated prisoners dilemma tournament where he showed tit-for-tat was a really good strategy: basically, to cooperate at first and then, on each future round, to do what your opponent did on the previous one.  I have a lot more to say on this topic, but I'll skip to Martin's talk.  Martin presented some interesting results where he showed us that if noise is introduced into this repeated game (like in real life), and one runs an evolutionary tournament that starts with random strategies, first tit-for-tat will evolve.  But then, tit-for-tat will be taken over by a more forgiving versions tit-for-tat, which will subsequently be dominated by completely cooperative strategies, which will be exploited by completely defecting strategies, which will then be taken over by tit-for-tats again.  And this cycle repeats!  Martin pessimistically suggested this this cycle is inevitable in the real world as well.  Of course, his talk had a lot more, so if you want to know the details of this and more, I recommend you watch it!

possible prisoner's dilemma payoff matrix, image from "beyond intractability"

Then, Hatmut Neven, of Google talked about the machine learning behind "Google Goggles," the reverse-image search where you take a picture of an object, send it to Google as a query, and Google will hopefully return some relevant results.  I hadn't thought much about this reverse-search problem, so a lot of the talk was interesting to me on a basic level.  For example, if you send a picture of a fancy chair, it's useless for Google to tell you it's a chair, you're probably looking to know what type of chair it is so you can buy it -- this makes the problem very hard.  But it doesn't mean Hatmut and his team haven't made a scary amount of progress: Hatmut says that Google can, given a picture of an arbitrary person who has about a dozen photos of them in Google's databases (i.e. online), correctly classify them in the top ten results.  Google hasn't activated this feature for privacy reasons, but if (or when?) Google does, this means that you could pretty much see someone on the street, take a picture of them, and find out who they are (and they don't even have to be a famous actress).  Hatmut thinks that in 10 years, we'll be able to carry a camera with us, which will, without any special querying, know more about the world around us than we do -- it will know what species of trees are around us, the origin of the name of the street we are on, the history of the author of the book we're reading, etc. Exciting stuff.  Finally, Hatmut talked about how quantum computing might help with all this, and he gave a much more positive endorsement of D-Wave than I'd read elsewhere.

D-Wave's 128-bit chip, image from Wikipedia

The last talk was by David Ferrucci about the IBM Watson system that beat the top two humans on Jeopardy (see my post on Watson).  He talked about the immense challenge of building Watson, the collaboration among many different types of researchers, the scientific problems involved, and the various problems his team had to overcome.  The task of building Watson was so monumental that many (non computer scientist) people couldn't get their heads around it.  David described an incident when he went on a radio show to talk about Watson and was asked by the radio host "So, you're going to give the computer all the questions and answers, and when a question is asked, the computer will pull up the right answer?"  David said, surely rolling his eyes, "No. We don't know any of the answers in advance, let alone the questions."  Then the radio show host responded "I don't understand. How are you going to do it then?" "Exactly!" said David.  I trust I'm not spoiling anything if I tell you machine learning was a big part of the solution.

Watson's avatar, image from Wikipedia

Neural Networks 

John Langford (who hosted my postdoc at Yahoo! Research) already blogged that neural networks were making a comeback, so to speak, at ICML 2011.  In addition to the sheer number of "deep learning" papers, lots of people excitedly talked about what great things they could train their neural nets to do.  I would add to John's post, after talking to Bill and Ilya, that another reason this area is becoming "hot" is that we finally have the computing power to train neural networks for real-world problems, something that was previously much more difficult.

I must admit that I don't know enough about neural networks, and the reason is that it's not easy to see how the fit into the larger world from a learning theory perspective.  For example, we know that a single-layer neural network cannot learn even the XOR function, that recurrent neural nets can simulate Turing Machines, and that certain classes of neural nets are not properly PAC learnable.  But from what I can see, the theory of neural nets is of a very different flavor than, say, the theory of decision trees.  If someone who knows these connections better wrote up a survey, I'm sure I'm not the only one who would be grateful. (Even better, maybe such a survey exists and I'm not aware of it.)

Papers I Liked

Given that ICML has 4 or 5 parallel tracks, I didn't get to see even 20% of the talks, but I tried to catch what I could at the poster sessions.  Here are a few non neural-networks (for the neural networks ones, see John's post) papers I liked, besides my own, of course.

"On the Necessity of Irrelevant Variables" by David Helmbold and Phil Long.  The main gist of this paper is that irrelevant variables don't hurt a predictor nearly as much as relevant variables help.  So if you're trying to do feature selection and aren't sure whether to add a feature or not, add it.

"Clustering Partially Observed Graphs via Covex Optimization" by Ali Jalali, Yudong Chen, Sujay Sanghavi, and Huan Xu.  Here, they do correlation clustering with partial observations, which is NP-Hard, so instead of the usual approach of making an approximation algorithm, they give an algorithm which succeeds of the data satisfies certain properties.  They're not the first to do this type of analysis, but I rather liked their type of data-dependent assumption.

"Optimal Distributed Online Prediction" by Ofer Dekel, Ran Gilad-Bachrach, Ohad Shamir, and Lin Xiao.  This paper considers online prediction, but in a distributed model, where one can get speedups, possibly at some cost in the regret.  They derive optimal regret bounds for convex losses and i.i.d. examples, and show that for many parameter settings,  you can actually get the same asymptotic regret as without parallelizing.

"Doubly Robust Policy Evaluation and Learning" by Miroslav Dudik, John Langford, and Lihong Li.  I'm a bit biased about this paper because I was at Yahoo! when they were woking on it. They basically show a technique for evaluating a new bandit policy on historical data, where you can get an unbiased evaluation as long as you have either a good model of rewards or a good model of the past policy. Hence, doubly robust.

Overall

ICML 2011 was great.  I'm looking forward to ICML 2012! One extra bonus is that ICML 2012 will be co-located in Edinburgh with COLT 2012, the main learning theory conference.

Update (7/26/11): Videos of all the talks from ICML 2011 have now been posted here.

If you're interested in machine learning, you should follow John Langford's blog, hunch.net.  Also, John will co-chair ICML 2012.

My Thoughts on NIPS 2010

This year, I attended my first NIPS conference -- NIPS stands for "Neural Information Processing Systems," but it has become one of the main venues for machine learning research in general. I've attended lots of machine learning conferences before, but somehow never managed to go to NIPS. This year, I had run out of excuses.

NIPS is broader than ICML, the other general machine learning conference, and it is certainly much broader than COLT and ALT, the learning theory conferences. I thought this would be a bad thing, as I expected that I wouldn't understand many of the talks and posters, but it turned out to have been one of my favorite (if not my favorite) conference experiences.

First, because most people at NIPS are in the position of not being familiar with everything, there wasn't the expectation that I would immediately know the model or the problem being tackled when I talked to someone. This strangely made things more, not less, accessible. Also, because lots of topics were outside my areas of research interests, I didn't feel internal pressure to attend many of the talks, and it allowed me to relax and have more time to talk to other researchers. And because NIPS draws a huge attendance, I got to reconnect with many people I hadn't seen for a while, and I had a chance to meet people whom I know only through their research or through correspondence.

On top of all that, within my areas of research, there were quite a few really good contributions, and I wanted to point out some of the papers that I found especially interesting. This is, of course, a list biased not only toward my interests, but also toward the papers whose presentations I happened to catch.

• A Theory of Multiclass Boosting by I. Mukherjee, R.E. Schapire. This paper characterizes necessary and sufficient conditions for multiclass boosting and gives a new multiclass algorithm not based on reductions to the binary case. A nice an elegant paper, it won one of the best student paper awards.
• Online Learning: Random Averages, Combinatorial Parameters, and Learnability by A. Rakhlin, K. Sridharan, A. Tewari. Building on the work of [BPS '09], this paper extends various notions of complexity to the online setting (Radamacher complexity, covering numbers, fat shattering dimension) and proves general online learnability guarantees based on these notions. This is similar in flavor to already known general offline guarantees.
• Trading off Mistakes and Don’t-Know Predictions by A. Sayedi, M. Zadimoghaddam, A. Blum. This paper analyzes a model where the learner is allowed a limited number of prediction mistakes, but is also allowed to answer "I don't know," generalizing the KWIK model [LLW '08]. They prove some nice trade-offs, and there seem to be many potential interesting extensions.
  
• Learning from Logged Implicit Exploration Data by A. Strehl, J. Langford, L. Li, S. Kakade. This paper discusses how to analyze the performance of a new contextual bandit algorithm by running it on previously recorded data. It's pretty intuitive that this should be possible, and this paper goes through the math carefully. This is a problem confronted by many content-serving companies, and I imagine this analysis will be quite useful.
  

Additionally, I also enjoyed David Parkes's talk on the interactions between machine learning and mechanism design -- this seems to be a very interesting area of research.

Finally, I also attended the NIPS workshops, which were rumored to be quite good, and they certainly lived up to their expectations. I especially enjoyed the workshop on Computational Social Science and Wisdom of the Crowds -- I decided to attend a workshop in an area about which I know very little, and all the talks were really good. In fact, from Yiling Chen's and Jake Abernathy's talks, I even learned about connections between prediction markets and online learning, so this workshop ended up being more closely related to my research than I expected.

Clearly, I've been missing out by not having attended the previous NIPS meetings. I'm planning to make up for it in future years.