Machine Learning (Theory)


CNTK and Vowpal Wabbit tutorials at NIPS

Both CNTK and Vowpal Wabbit have pirate tutorials at NIPS. The CNTK tutorial is 1 hour during the lunch break of the Optimization workshop while the VW tutorial is 1 hour during the lunch break of the Extreme Multiclass workshop. Consider dropping by either if interested.

CNTK is a deep learning system started by the speech people who started the deep learning craze and grown into a more general platform-independent deep learning system. It has various useful features, the most interesting of which is perhaps efficient scalable training. Using GPUs with allreduce and one-bit sgd it achieves both high efficiency and scalability over many more GPUs than could ever fit into a single machine. This capability is unique amongst all open deep learning codebases so everything else looks nerfed in comparison. CNTK was released in April so this is the first chance for many people to learn about it.

The Vowpal Wabbit tutorial just focuses on what is new this year.

  1. The learning to search framework has greatly matured and is now easily used to solve ad-hoc joint(structured) prediction problems. The ICML tutorial covers algorithms/analysis so this is about using the system.
  2. VW has also become the modeling element of a larger system (called the decision service) which gathers data and uses it as per Contextual Bandit learning. This is now generally usable, and is the first general purpose system of this sort.


ICML 2016 in NYC and KDD Cup 2016

ICML 2016 is in New York City. I expect it to be the largest ICML by far given the destination—New York is the place which is perhaps easiest to reach from anywhere in the world and it has the largest machine learning meetup anywhere in the world.

I am the general chair this year, which is light in work but heavy in responsibilities. Some things I worry about:

  1. How many people will actually come? Numbers are difficult to guess with the field growing and the conference changing locations. I believe we need capacity for at least 3000 people based on everything I know.
  2. New York is expensive. What can be done about it? One thought is that we should actively setup a roommate finding system so the costs of hotels can be shared. Up to 3 people can share a hotel room for the conference hotel (yes, each with their own bed), and that makes the price much more reasonable. I’m also hoping donations will substantially defray the cost. If others have creative ideas, I’m definitely interested.

Markus Weimer also points out the 2016 KDD Cup which has a submission deadline of December 6. KDD Cup datasets have become common reference for many machine learning papers, so this is a good way to get your problem solved well by many people.


Randomized experimentation

One good thing about doing machine learning at present is that people actually use it! The back-ends of many systems we interact with on a daily basis are driven by machine learning. In most such systems, as users interact with the system, it is natural for the system designer to wish to optimize the models under the hood over time, in a way that improves the user experience. To ground the discussion a bit, let us consider the example of an online portal, that is trying to present interesting news stories to its user. A user comes to the portal and based on whatever information the portal has on the user, it recommends one (or more) news stories. The user chooses to read the story or not and life goes on. Naturally the portal wants to better tailor the stories it displays to the users’ taste over time, which can be observed if users start to click on the displayed story more often.

A natural idea would be to use the past logs and train a machine learning model which prefers the stories that users click on and discourages the stories which are avoided by the users. This sounds like a simple classification problem, for which we might use an off-the-shelf algorithm. This is indeed done reasonably often, and the offline logs suggest that the newly trained model will result in a lot more clicks than the old one. The new model is deployed, only to find out its performance is not as good as hoped, or even poorer than what was happening before! What went wrong? The natural reaction is typically that (a) the machine learning algorithm needs to be improved, or (b) we need better features, or (c) we need more data. Alas, in most of these cases, the right answer is (d) none of the above. Let us see why this is true through a simple example.

Imagine a simple world where some of our users are from New York and others are from Seattle. Some of our news stories pertain to finance, and others pertain to technology. Let us further imagine that the probability of a click (henceforth CTR for clickthrough rate) on a news article based on city and subject has the following distribution:


Finance CTR

Tech CTR

New York 1 0.6
Seattle 0.4 0.79

Table1: True (unobserved) CTRs

Of course, we do not have this information ahead of time while designing the system, so our starting system recommends articles according to some heuristic rule. Imagine that we user the rule:

  • New York users get Tech stories, Seattle users get Finance stories.

Now we collect the click data according to this system for a while. As we obtain more and more data, we obtain increasingly accurate estimates of the CTR for Tech stories and NY users, as well as Finance stories and Seattle users (0.6 and 0.4 resp.). However, we have no information on the other two combinations. So if we train a machine learning algorithm to minimize the squared loss between predicted CTR on an article and observed CTR, it is likely to predict the average of observed CTRs (that is 0.5) in the other two blocks. At this point, our guess looks like:



Finance CTR

Tech CTR

New York 1 / ? / 0.5 0.6 / 0.6 / 0.6
Seattle 0.4 / 0.4 / 0.4 0.79 / ? / 0.5

Table2: True / observed / estimated CTRs

Note that this would be the case even with infinite data and an all powerful learner, so machine learning is not to be faulted in any way here. Given these estimates, we naturally realize that show finance articles to Seattle users was a mistake, and switch to Tech. But Tech is also looking pretty good in NY, and we stick with it. Our new policy is:

  • Both NY and Seattle users get Tech articles.

Running the new system for a while, we will fix the erroneous estimates for the Tech CTR on Seattle (that is, up 0.5 to 0.79). But we still have no signal that makes us prefer Finance over Tech in NY. Indeed even with infinite data, the system will be stuck with this suboptimal choice at this point, and our CTR estimates will look something like:


Finance CTR

Tech CTR

New York 1 / ? / 0.59 0.6 / 0.6 / 0.6
Seattle 0.4 / 0.4 / 0.4 0.79 / 0.79 / 0.79

Table3: True / observed / estimated CTRs

We can now assess the earlier claims:

  1. More data does not help: Since Observed and True CTRs match wherever we are collecting data
  2. Better learning algorithm does not help: Since Predicted and Observed CTRs coincide wherever we are collecting data
  3. Better data does help!! We should not be having the blank cell in observed column.

This seems simple enough to fix though. We should have really known better than to completely omit observations in one cell of our table. With good intentions, we decide to collect data in all cells. We choose to use the following rule:

  • Seattle users get Tech stories during day and finance stories during night
  • Similarly, NY users get Tech stories during day and finance stories during night

We are now collecting data on each cell, but we find that our estimates still lead us to a suboptimal policy. Further investigation might reveal that users are more likely to read finance stories during the day when the markets are open. So when we only display finance stories during night, we underestimate the finance CTR and end up with wrong estimates. Realizing the error of our ways, we might try to fix this again and then run into another problem and so on.

The issue we have discovered above is that of confounding variables. There is lot of wonderful work and many techniques that can be used to circumvent confounding variables in experimentation. Here, I mention the simplest one and perhaps the most versatile one of them: Randomization. The idea is that instead of recommending stories to users according to a fix deterministic rule, we allow for different articles to be presented to the user according to some distribution. This distribution does not have to be uniform. In fact, good randomization would likely focus on plausibly good articles so as to not degrade the user experience. However, as long as we add sufficient randomization, we can then obtain consistent counterfactual estimates of quantities from our experimental data. There is growing literature on how to do this well. A nice paper which covers some of these techniques and provides an empirical evaluation is A more involved example in the context of computational advertising at Microsoft is discussed in




The NIPS experiment

Tags: Conferences,Machine Learning jl@ 2:38 pm

Corinna Cortes and Neil Lawrence ran the NIPS experiment where 1/10th of papers submitted to NIPS went through the NIPS review process twice, and then the accept/reject decision was compared. This was a great experiment, so kudos to NIPS for being willing to do it and to Corinna & Neil for doing it.

The 26% disagreement rate presented at the conference understates the meaning in my opinion, given the 22% acceptance rate. The immediate implication is that between 1/2 and 2/3 of papers accepted at NIPS would have been rejected if reviewed a second time. For analysis details and discussion about that, see here.

Let’s give P(reject in 2nd review | accept 1st review) a name: arbitrariness. For NIPS 2014, arbitrariness was ~60%. Given such a stark number, the primary question is “what does it mean?”

Does it mean there is no signal in the accept/reject decision? Clearly not—a purely random decision would have arbitrariness of ~78%. It is however quite notable that 60% is much closer to 78% than 0%.

Does it mean that the NIPS accept/reject decision is unfair? Not necessarily. If a pure random number generator made the accept/reject decision, it would be ‘fair’ in the same sense that a lottery is fair, and have an arbitrariness of ~78%.

Does it mean that the NIPS accept/reject decision could be unfair? The numbers give no judgement here. It is however a natural fallacy to imagine that random judgements derived from people implies unfairness, so I would encourage people to withhold judgement on this question for now.

Is an arbitrariness of 0% the goal? Achieving 0% arbitrariness is easy: just choose all papers with an md5sum that ends in 00 (in binary). Clearly, there is something more to be desired from a reviewing process.

Perhaps this means we should decrease the acceptance rate? Maybe, but this makes sense only if you believe that arbitrariness is good, as it will almost surely increase the arbitrariness. In the extreme case where only one paper is accepted, the odds of it being the rejected on re-review are near 100%.

Perhaps this means we should increase the acceptance rate? If all papers submmitted were accepted, the arbitrariness would be 0, but as mentioned above arbitrariness 0 is not the goal.

Perhaps this means that NIPS is a very broad conference with substantial disagreement by reviewers (and attendees) about what is important? Maybe. This even seems plausible to me, given anecdotal personal experience. Perhaps small highly-focused conferences have a smaller arbitrariness?

Perhaps this means that researchers submit themselves to an arbitrary process for historical reasons? The arbitrariness is clear, but the reason less so. A mostly-arbitrary review process may be helpful in the sense that it gives authors a painful-but-useful opportunity to debug the easy ways to misinterpret their work. It may also be helpful in that it perfectly rejects the bottom 20% of papers which are actively wrong, and hence harmful to the process of developing knowledge. None of these reasons are confirmed of course.

Is it possible to do better? I believe the answer is “yes”, but it should be understood as a fundamentally difficult problem. Every program chair who cares tries to tweak the reviewing process to be better, and there have been many smart program chairs that tried hard. Why isn’t it better? There are strong nonvisible constraints on the reviewers time and attention.

What does it mean? In the end, I think it means two things of real importance.

  1. The result of the process is mostly arbitrary. As an author, I found rejects of good papers very hard to swallow, especially when the reviews were nonsensical. Learning to accept that the process has a strong element of arbitrariness helped me deal with that. Now there is proof, so new authors need not be so discouraged.
  2. CMT now has a tool for measuring arbitrariness that can be widely used by other conferences. Joelle and I changed ICML 2012 in various ways. Many of these appeared beneficial and some stuck, but others did not. In the long run, it’s the things which stick that matter. Being able to measure the review process in a more powerful way might be beneficial in getting good review practices to stick.

Other commentary from Lance, Bert, and Yisong.

Edit: Cross-posted on CACM.


Vowpal Wabbit 7.8 at NIPS

I just created Vowpal Wabbit 7.8, and we are planning to have an increasingly less heretical followup tutorial during the non-“ski break” at the NIPS Optimization workshop. Please join us if interested.

I always feel like things are going slow, but in the last year, but there have been many changes overall. Notes for 7.7 are here. Since then, there are several areas of improvement as well as generalized bug fixes and refactoring.

  1. Learning to Search: Hal completely rewrote the learning to search system, enough that the numbers here are looking obsolete. Kai-Wei has also created several advanced applications for entity-relation and dependency parsing which are promising.
  2. Languages Hal also created a good python library, which includes call-backs for learning to search. You can now develop advanced structured prediction solutions in a nice language. Jonathan Morra also contributed an initial Java interface.
  3. Exploration The contextual bandit subsystem now allows evaluation of an arbitrary policy, and an exploration library is now factored out into an independent library (principally by Luong with help from Sid and Sarah). This is critical for real applications because randomization must happen at the point of decision.
  4. Reductions The learning reductions subsystem has continued to mature, although the perfectionist in me is still dissatisfied. As a consequence, it’s now pretty easy to program new reductions, and the efficiency of these reductions has generally improved. Several news ones are cooking.
  5. Online Learning Alekh added an online SVM implementation based on LaSVM. This is known to parallelize well via the para-active approach.

This project has grown quite a bit—there are about 30 different people contributing to VW since the last release, and there is now a VW meetup (December 8th!) in the bay area that I wish I could attend.

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