Reinforcement Learning, also referred to as Online Learning, is an adaptive way to select between versions of a product or advertisement to optimize a desired user response.
The Multi-Armed Bandit Problem
The multi-armed bandit problem has been explored by statisticians for decades. In the time of its origin, one-armed bandit was common term for a Slot Machine. It makes sense, then, that the premise of this problem involves a slot machine with many arms of varying payout rates/probabilities. The challenge for analysts is to systematically and sensibly sample and make observations on the outcomes from different arms on the machine to maximize the probable payout without wasting finite investment resources.
For the modern era, this problem is often approached from an advertising perspective. This tutorial laid out the following scenario: an online marketing company needs to choose between 10 versions of an advertisement for their product. This is a multi-armed bandit problem because there is no objective/available data on click-through rates until the ads are piloted, however it would not be wise for them to invest equally on all ads as clickthrough trends begin to emerge. Therefore, we use an adaptive Reinforcement Learning approach to narrow the field down to the top candidate(s), and to do so as quickly as possible.
Upper Confidence Bound (UCB)
Our analysis of the multi-armed bandit problem in R began with a Random Sampling for a baseline. As expected, there was little variation in the clickthrough results for this method. By contrast, the Upper Confidence Bound algorithm showed a clear top contender that was almost exclusively called in the final hundreds of iterations (out of 10,000). The total reward for the UCB algorithm was approximately 2200 – about 1000 more total clickthroughs than in the Random Sampling algorithm.
Thompson Sampling showed an even greater improvement than the UCB model. It more than doubled the reward given by the random sampling model – from 1200 in Random Sampling to about 2600 from Thompson Sampling. While searching for additional insights on this method, I found A Tutorial on Thompson Sampling by Daniel Russo et al. (2018), which I would like to revisit for a more technical explanation of the algorithm once my graduate and independent coursework calms down a bit.