Bandit problems, Markov decision processes, a smattering of dynamic programming, game theory, optimal control, and online learning the solutions to such problems, esp. reinforcement learning.
Learning, where you must learn an optimal action in response to your stimulus, possibly an optimal policy of trying different actions over time, not just an MMSE-minimal prediction from complete data.
Comes in adversarial and stochastic flavours, apparently, although I’ve hit the boundaries of my knowledge there.
See clickbait bandits.
Basics: RL versus supervised learning
Richard Xu’s take:
- No data label as in supervised learning, only a reward signal
- feedback is delayed
- data are not i.i.d. but sequentially dependent
- agent’s actions affect the data it receives.
There’s interesting mathematics! social graphs! Self-exciting point processes! And all the bandit problem literature!
New tool by Microsoft: Multi-World Testing (MWT) appears to be an online learning problem that augments its data by re-using the data for offline testing:
… is a toolbox of machine learning technology for principled and efficient experimentation, plausibly applicable to most Microsoft services that interact with customers. In many scenarios, this technology is exponentially more efficient than the traditional A/B testing. The underlying research area, mature and yet very active, is known under many names: “multi-armed bandits”, “contextual bandits”, “associative reinforcement learning”, and “counterfactual evaluation”, among others.
To take an example, suppose one wants to optimize clicks on suggested news stories. To discover what works, one needs to explore over the possible news stories. Further, if the suggested news story can be chosen depending on the visitor’s profile, then one needs to explore over the possible “policies” that map profiles to news stories (and there are exponentially more “policies” than news stories!). Traditional ML fails at this because it does not explore. Whereas MWT allows you to explore continuously, and optimize your decisions using this exploration data.
Removing the credit assignment problem from reinforcement learning yields the Contextual Bandit setting which we know is generically solvable in the same manner as common supervised learning problems. I know of about a half-dozen real-world successful contextual bandit applications typically requiring the cooperation of engineers and deeply knowledgeable data scientists.
Can we make this dramatically easier? We need a system that explores over appropriate choices with logging of features, actions, probabilities of actions, and outcomes. These must then be fed into an appropriate learning algorithm which trains a policy and then deploys the policy at the point of decision. Naturally, this is what we’ve done and now it can be used by anyone. This drops the barrier to use down to: “Do you have permissions? And do you have a reasonable idea of what a good feature is?”
A key foundational idea is Multiworld Testing: the capability to evaluate large numbers of policies mapping features to action in a manner exponentially more efficient than standard A/B testing. This is used pervasively in the Contextual Bandit literature and you can see it in action for the system we’ve made at Microsoft Research.
- Sébastien Bubeck ‘s intro (complements and update BuCe12)
- Sergey Feldman, Bandits for Recommendation Systapplicatoinems is an EZ-introduction.
- Microsoft’s recommended intro is BuCe12.
- Richard S. Sutton and Andrew G. Barto’s Reinforcement Learning: An Introduction
- Langford’s 2013 NIPS presentation (Lang13)
- Elad Hazan’s origin story for online convex optimisation has just made me realise how new it is. It’s a nice complement to…
- …`Elad Hazan and Satyan Kale’s tutorial on online convex optimisation <http://www.cs.princeton.edu/~ehazan/tutorial/tutorial.htm>`_.
Bubeck again: Kernel-based methods for bandit convex optimization, part 1.
Ben Recht and Roy Frostig, Nesterov’s punctuation equilibrium:
In a landmark new paper by Salimans, Ho, Chen, and Sutskever from OpenAI, (SHCS17) the authors show that a particular class of genetic algorithms (called Evolutionary Strategies) gives excellent performance on a variety of reinforcement learning benchmarks. As optimizers, the application of genetic algorithms raises red flags and usually causes us to close browser windows. But fear not! As we will explain, the particular algorithm deployed happens to be a core method in optimization, and the fact that this method is successful sheds light on the peculiarities of reinforcement learning more than it does about genetic algorithms in general.
Pseudopolitical diversion: Clickbait bandit problems
An application, esp for contextual bandits.
There is a lot of stuff being done in bandit problems specifically to model consumers and web pages; Therefore I’ll talk about it from that perspective:
The science of treating consumers of modern news media like what they are, near-passive objects of surveillance and control. Because trying to rely on peoples’ rationality and agency to get things done has a poor track record in recent history.
The “bandit problems” phrase comes, by the way, from an extension of the “one armed bandit”, the poker machine, into a mathematical model for exploring the world by pulling on the arms of a poker machine.
Pseudopolitical diversion: There is a pleasing symmetry in that modern poker machines, and indeed the internet in general, model the customer as a metaphorical poker machine upon whose arm they pull to get a reward, and that this reward is addicting the customer to pulling on the arms of their literal poker machine. It’s a two-way battle of algorithms, but one side does not update its learning algorithms.
You should read this next one before you blame someone (especially a millennial, especially if you are not a millennial) for having no attention span, then take a deep look into your soul; Michael Schulson, if the internet is addictive, why don’t we regulate it?
As a consultant to Silicon Valley startups, Eyal helps his clients mimic what he calls the ‘narcotic-like properties’ of sites such as Facebook and Pinterest. His goal, Eyal told Business Insider, is to get users ‘continuing through the same basic cycle. Forever and ever.’
[…] For a tech company in the attention economy, the longer you’re engaged by variable rewards, the more time you spend online, and the more money they make through ad revenue.
Yet we keep blaming people.
Stupid rats, running the mazes we set them instead of dotcom startups.
Markov decision problems
Bellman and Howard’s classic discrete time control stochastic problem
- Warren Powell’s Introduction to markov decision processes
Too many CPU cycles? Why not look at optimal control in this setting?
“A POMDP is a partially observable Markov decision process. It is a model, originating in the operations research (OR) literature, for describing planning tasks in which the decision maker does not have complete information as to its current state. The POMDP model provides a convenient way of reasoning about tradeoffs between actions to gain reward and actions to gain information.”
VW contains a contextual bandit module which allows you to optimize a predictor based on already collected contextual bandit data. In other words, the module does not handle the exploration issue, it assumes it can only use the currently available data previously collected from some “exploration” policy.
Sequential surrogate interactive model optimisation
Not what I think of here, but it and many other hyperparameter optimization-type problems have an RL interpretation. That is, you can use RL to learn the hyper parameters of your deep learning model. (This is not the same as deep learning of RL policies.) See Sequential surrogate model optimisation for more of that, or the papers JaTa15, LJDR16a, LJDR16b, FBBZ17.
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