Dynamic Pricing with Multi-Armed Bandit: Learning by Doing | by Massimiliano Costacurta | Aug, 2023

Applying Reinforcement Learning strategies to real-world use cases, especially in dynamic pricing, can reveal many surprises

Massimiliano Costacurta
Towards Data Science
Photo by Markus Spiske on Unsplash

In the vast world of decision-making problems, one dilemma is particularly owned by Reinforcement Learning strategies: exploration versus exploitation. Imagine walking into a casino with rows of slot machines (also known as “one-armed bandits”) where each machine pays out a different, unknown reward. Do you explore and play each machine to discover which one has the highest payout, or do you stick to one machine, hoping it’s the jackpot? This metaphorical scenario underpins the concept of the Multi-armed Bandit (MAB) problem. The objective is to find a strategy that maximizes the rewards over a series of plays. While exploration offers new insights, exploitation leverages the information you already possess.

Now, transpose this principle to dynamic pricing in a retail scenario. Suppose you are an e-commerce store owner with a new product. You aren’t certain about its optimal selling price. How do you set a price that maximizes your revenue? Should you explore different prices to understand customer willingness to pay, or should you exploit a price that has been performing well historically? Dynamic pricing is essentially a MAB problem in disguise. At each time step, every candidate price point can be seen as an “arm” of a slot machine and the revenue generated from that price is its “reward.” Another way to see this is that the objective of dynamic pricing is to swiftly and accurately measure how a customer base’s demand reacts to varying price points. In simpler terms, the aim is to pinpoint the demand curve that best mirrors customer behavior.

In this article, we’ll explore four Multi-armed Bandit algorithms to evaluate their efficacy against a well-defined (though not straightforward) demand curve. We’ll then dissect the primary strengths and limitations of each algorithm and delve into the key metrics that are instrumental in gauging their performance.

Traditionally, demand curves in economics describe the relationship between the price of a product and the quantity of the product that consumers are willing to buy. They generally slope downwards, representing the common observation that as price rises, demand typically falls, and vice-versa. Think of popular products such as smartphones or concert tickets. If prices are lowered, more people tend to buy, but if prices skyrocket, even the ardent fans might think twice.

Yet in our context, we’ll model the demand curve slightly differently: we’re putting price against probability. Why? Because in dynamic pricing scenarios, especially digital goods or services, it’s often more meaningful to think in terms of the likelihood of a sale at a given price than to speculate on exact quantities. In such environments, each pricing attempt can be seen as an exploration of the likelihood of success (or purchase), which can be easily modeled as a Bernoulli random variable with a probability p depending on a given test price.

Here’s where it gets particularly interesting: while intuitively one might think the task of our Multi-armed Bandit algorithms is to unearth that ideal price where the probability of purchase is highest, it’s not quite so straightforward. In fact, our ultimate goal is to maximize the revenue (or the margin). This means we’re not searching for the price that gets the most people to click ‘buy’ — we’re searching for the price that, when multiplied by its associated purchase probability, gives the highest expected return. Imagine setting a high price that fewer people buy, but each sale generates significant revenue. On the flip side, a very low price might attract more buyers, but the total revenue might still be lower than the high price scenario. So, in our context, talking about the ‘demand curve’ is somewhat unconventional, as our target curve will primarily represent the probability of purchase rather than the demand directly.

Now, getting to the math, let’s start by saying that consumer behavior, especially when dealing with price sensitivity, isn’t always linear. A linear model might suggest that for every incremental increase in price, there’s a constant decrement in demand. In reality, this relationship is often more complex and nonlinear. One way to model this behavior is by using logistic functions, which can capture this nuanced relationship more effectively. Our chosen model for the demand curve is then:

Here, a denotes the maximum achievable probability of purchase, while b modulates the sensitivity of the demand curve against price changes. A higher value of b means a steeper curve, approaching more rapidly to lower purchase probabilities as the price increases.

Four examples of demand curves with different combinations of parameters a and b

For any given price point, we’ll be then able to obtain an associated purchase probability, p. We can then input p into a Bernoulli random variable generator to simulate the response of a customer to a particular price proposal. In other words, given a price, we can easily emulate our reward function.

Next, we can multiply this function by the price in order to get the expected revenue for a given price point:

Unsurprisingly, this function does not reach its maximum in correspondence with the highest probability. Also, the price associated with the maximum does not depend on the value of the parameter a, while the maximum expected return does.

Expected revenue curves with related maxima

With some recollection from calculus, we can also derive the formula for the derivative (you’ll need to use a combination of both the product and the chain rule). It’s not exactly a relaxing exercise, but it’s nothing too challenging. Here is the analytical expression of the derivative of the expected revenue:

This derivative allows us to find the exact price that maximizes our expected revenue curve. In other words, by using this specific formula in tandem with some numerical algorithms, we can easily determine the price that sets it to 0. This, in turn, is the price that maximizes the expected revenue.

And this is exactly what we need, since by fixing the values of a and b, we’ll immediately know the target price that our bandits will have to find. Coding this in Python is a matter of a few lines of code:

For our use case, we’ll set a = 2 and b = 0.042, which will give us a target price of about 30.44, associated with an optimal probability of 0.436 ( → optimal average reward is 30.44*0.436=13.26). This price is obviously unknown in general and it is exactly the price that our Multi-armed Bandit algorithms will seek.

Now that we’ve identified our objectives, it’s time to explore various strategies for testing and analyzing their performance, strengths, and weaknesses. While several algorithms exist in MAB literature, when it comes to real-world scenarios, four primary strategies (along with their variations) predominantly form the backbone. In this section, we’ll provide a brief overview of these strategies. We assume the reader has a foundational understanding of them; however, for those interested in a more in-depth exploration, references are provided at the end of the article. After introducing each algorithm, we’ll also present its Python implementation. Although each algorithm possesses its unique set of parameters, they all commonly utilize one key input: the arm_avg_reward vector. This vector denotes the average reward garnered from each arm (or action/price) up to the current time step t. This critical input guides all the algorithms in making informed decisions about the subsequent price setting.

The algorithms I am going to apply to our dynamic pricing problem are the following:

Greedy: This strategy is like always going back to the machine that gave you the most coins the first few times you played. After trying out each machine a bit, it sticks with the one that seemed the best. But there might be a problem. What if that machine was just lucky at the start? The Greedy strategy might miss out on better options. On the bright side, the code implementation is really simple:

It’s essential to differentiate the initial scenario (when all rewards are 0) from the regular one. Often, you’ll find only the ‘else’ part implemented, which indeed works even when all rewards are at 0. Yet, this approach can lead to a bias toward the first element. If you make this oversight, you might end up paying that bias, particularly if the optimal reward happens to be tied to the first arm (yes, I’ve been there). The Greedy approach is typically the least-performing one and we’ll primarily use it as our performance baseline.

ϵ-greedy: The ε-greedy (epsilon-greedy) algorithm is a modification to tackle the main drawback of the greedy approach. It introduces a probability ε (epsilon), typically a small value, to select a random arm, promoting exploration. With a probability 1−ε, it chooses the arm with the highest estimated reward, favoring exploitation. By balancing between random exploration and exploitation of known rewards, the ε-greedy strategy aims to achieve better long-term returns compared to purely greedy methods. Again, the implementation is immediate, it’s simply an additional ‘if’ on top of the Greedy code.

UCB1 (Upper Confidence Bound): The UCB1 strategy is like a curious explorer trying to find the best restaurant in a new city. While there’s a favorite spot they’ve enjoyed, the allure of potentially discovering an even better place grows with each passing day. In our context, UCB1 combines the rewards of known price points with the uncertainty of those less explored. Mathematically, this balance is achieved through a formula: the average reward of a price point plus an “uncertainty bonus” based on how long since it was last tried. This bonus is calculated as

and represents the “growing curiosity” about the untried price. The hyperparameter C controls the balance between exploitation and exploration, with higher values of C encouraging more exploration of less-sampled arms. By always selecting the price with the highest combined value of known reward and curiosity bonus, UCB1 ensures a mix of sticking to what’s known and venturing into the unknown, aiming to uncover the optimal price point for maximum revenue. I’ll start with the by-the-book implementation of this approach, but we’ll soon see that we need to tweak it a bit.

Thompson Sampling: This Bayesian approach addresses the exploration-exploitation dilemma by probabilistically selecting arms based on their posterior reward distributions. When these rewards adhere to a Bernoulli distribution, representing binary outcomes like success/failure, Thompson Sampling (TS) employs the Beta distribution as a conjugate prior (see this table for reference). Initiating with a non-informative Beta(1,1) prior for every arm, the algorithm updates the distribution’s parameters upon observing rewards: a success increases the alpha parameter, while a failure augments the beta. During each play, TS draws from the current Beta distribution of each arm and opts for the one with the top sampled value. This methodology allows TS to dynamically adjust based on gathered rewards, adeptly balancing between the exploration of uncertain arms and the exploitation of those known to be rewarding. In our specific scenario, although the foundational reward function follows a Bernoulli distribution (1 for a purchase and 0 for a missed purchase), the actual reward of interest is the product of this basic reward and the current price under test. Hence, our implementation of TS will need a slight modification (which will also introduce some surprises).

The change is actually pretty simple: to determine the most promising next arm, samples extracted from the posterior estimates are multiplied by their respective price points (line 3). This modification ensures decisions are anchored on the anticipated average revenue, shifting the focus from the highest purchase probability.

At this point, having gathered all the key ingredients to construct a simulation comparing the performance of the four algorithms in our dynamic pricing context, we must ask ourselves: what exactly will we be measuring? The metrics we choose are pivotal, as they will guide us in the process of both comparing and improving the algorithm implementation. In this endeavor, I’m zeroing in on three key indicators:

  1. Regret: This metric measures the difference between the reward obtained by the chosen action and the reward that would have been obtained by taking the best possible action. Mathematically, regret at time t is given by: Regret(t)=Optimal Reward(t)−Actual Reward(t). Regret, when accumulated over time, provides insight into how much we’ve “lost” by not always choosing the best action. It is preferred over cumulative reward because it provides a clearer indication of the algorithm’s performance relative to the optimal scenario. Ideally, a regret value close to 0 indicates proximity to optimal decision-making.
  2. Reactivity: This metric gauges the speed at which an algorithm approaches a target average reward. Essentially, it’s a measure of the algorithm’s adaptability and learning efficiency. The quicker an algorithm can achieve the desired average reward, the more reactive it is, implying a swifter adjustment to the optimal price point. In our case the target reward is set at 95% of the optimal average reward, which is 13.26. However, initial steps can exhibit high variability. For instance, a lucky early choice might result in a success from a low probability arm associated with a high price, quickly achieving the threshold. Due to such fluctuations, I’ve opted for a stricter definition of reactivity: the number of steps required to attain 95% of the optimal average reward ten times, excluding the initial 100 steps.
  3. Arms Allocation: This indicates the frequency with which each algorithm utilizes the available arms. Presented as a percentage, it reveals the algorithm’s propensity to select each arm over time. Ideally, for the most efficient pricing strategy, we’d want an algorithm to allocate 100% of its choices to the best-performing arm and 0% to the rest. Such an allocation would inherently lead to a regret value of 0, denoting optimal performance.

Evaluating MAB algorithms poses challenges due to the highly stochastic nature of their outcomes. This means that because of the inherent randomness in determining quantities, the results can greatly vary from one run to another. For a robust evaluation, the most effective approach is to execute the target simulation multiple times, accumulate the results and metrics from each simulation, and then compute the average.

The initial step involves creating a function to simulate the decision-making process. This function will implement the feedback loop represented in the below image.

Feedback loop implemented in the simulation function

This is the implementation of the simulation loop:

The inputs to this function are:

  • prices: A list of candidate prices we wish to test (essentially our “arms”).
  • nstep: The total number of steps in the simulation.
  • strategy: The algorithm we aim to test for making decisions on the next price.

Lastly, we need to write the code for the outer loop. For every target strategy, this loop will call run_simulation multiple times, collect and aggregate the results from each execution, and then display the outcomes.

For our analysis, we’ll use the following configuration parameters:

  • prices: Our price candidates → [20, 30, 40, 50, 60]
  • nstep: Number of time steps for every simulation → 10000
  • nepoch: Number of simulation executions → 1000

Furthermore, by setting our price candidates, we can promptly obtain the associated purchase probabilities, which are (approximately) [0.60, 0.44, 0.31, 0.22, 0.15].

After running the simulation we are finally able to see some results. Let’s start from the plot of the cumulative regret:

From the graph, we can see that TS is the winner in terms of mean cumulative regret, but it takes around 7,500 steps to surpass ε-greedy. On the other hand, we have a clear loser, which is UCB1. In its basic configuration, it essentially performs on par with the greedy approach (we’ll get back to this later). Let’s try to understand the results better by exploring the other available metrics. In all four cases, the reactivity shows very large standard deviations, so we’ll focus on the median values instead of the means, as they are more resistant to outliers.

The initial observation from the plots reveals that while TS surpasses ε-greedy in terms of the mean, it slightly lags behind in terms of the median. However, its standard deviation is smaller. Particularly interesting is the reactivity bar plot, which shows how TS struggles to rapidly achieve a favorable average reward. At first, this was counterintuitive to me, but the mechanism behind TS in this scenario clarified matters. We previously mentioned that TS estimates purchase probabilities. Yet, decisions are made based on the product of these probabilities and the prices. Having knowledge of the real probabilities (that, as mentioned, are [0.60, 0.44, 0.31, 0.22, 0.15]) allows us to calculate the expected rewards TS is actively navigating: [12.06, 13.25, 12.56, 10.90, 8.93]. In essence, although the underlying probabilities differ considerably, the expected revenue values are relatively close from its perspective, especially in proximity to the optimal price. This means TS requires more time to discern the optimal arm. While TS remains the top-performing algorithm (and its median eventually drops below that of the ε-greedy one if the simulation is prolonged), it demands a longer period to identify the best strategy in this context. Below, the arm allocation pies show how TS and ε-greedy do pretty well in identifying the best arm (price=30) and using it most of the time during the simulation.

Now let’s get back to UCB1. Regret and reactivity confirm that it’s basically acting as a fully exploitative algorithm: quick to get a good level of average reward but with big regret and high variability of the outcome. If we look at the arm allocations that’s even more clear. UCB1 is only slightly smarter than the Greedy approach because it focuses more on the 3 arms with higher expected rewards (prices 20, 30, and 40). However, it essentially doesn’t explore at all.

Enter hyperparameter tuning. It’s clear that we need to determine the optimal value of the weight C that balances exploration and exploitation. The first step is to modify the UCB1 code.

In this updated code, I’ve incorporated the option to normalize the average reward before adding the “uncertainty bonus”, which is weighted by the hyperparameter C. The rationale for this is to allow for a consistent search range for the best hyperparameter (say 0.5–1.5). Without this normalization, we could achieve similar results, but the search interval would need adjustments based on the range of values we’re dealing with each time. I’ll spare you the boredom of finding the best C value; it can be easily determined through a grid search. It turns out that the optimal value is 0.7. Now, let’s rerun the simulation and examine the results.

That’s quite the plot twist, isn’t it? Now, UCB1 is clearly the best algorithm. Even in terms of reactivity, it has only slightly deteriorated compared to the previous score.

Additionally, from the perspective of arm allocation, UCB1 is now the undisputed leader.

  • Theory vs. Experience: Starting with book-based learning is an essential first step when delving into new topics. However, the sooner you immerse yourself in hands-on experiences, the faster you’ll transform information into knowledge. The nuances, subtleties, and corner cases you encounter when applying algorithms to real-world use cases will offer insights far beyond any data science book you might read.
  • Know Your Metrics and Benchmarks: If you can’t measure what you’re doing, you can’t improve it. Never begin any implementations without understanding the metrics you intend to use. Had I only considered regret curves, I might have concluded, “UCB1 doesn’t work.” By evaluating other metrics, especially arm allocation, it became evident that the algorithm simply wasn’t exploring sufficiently.
  • No One-Size-Fits-All solutions: While UCB1 emerged as the top choice in our analysis, it doesn’t imply it’s the universal solution for your dynamic pricing challenge. In this scenario, tuning was straightforward because we knew the optimal value we sought. In real life, situations are never so clear-cut. Do you possess enough domain knowledge or the means to test and adjust your exploration factor for the UCB1 algorithm? Perhaps you’d lean towards a reliably effective option like ε-greedy that promises immediate results. Or, you might be managing a bustling e-commerce platform, showcasing a product 10000 times per hour, and you’re willing to be patient, confident that Thompson Sampling will attain the maximum cumulative reward eventually. Yeah, life ain’t easy.

Finally, let me say that if this analysis seemed daunting, unfortunately, it already represents a very simplified situation. In real-world dynamic pricing, prices and purchase probabilities don’t exist in a vacuum — they actually exist in ever-changing environments and they’re influenced by various factors. For example, it’s highly improbable that purchase probability remains consistent throughout the year, across all customer demographics and regions. In other words, to optimize pricing decisions, we must consider our customers’ contexts. This consideration will be the focal point of my next article, where I’ll delve deeper into the problem by integrating customer information and discussing Contextual Bandits. So, stay tuned!

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This post originally appeared on TechToday.