Have you been pondering upon whether to opt for Multi-Armed Bandit or not? In this webinar, VWO’s Data Scientists Ishan Goel and Anshul Gupta take you through what it is, when to use it and when not, A/B testing vs. MAB, and more.

2. What are Multi-armed Bandits?
What ARE THEY? Why ARE WE TALKING
ABOUT THEM?
Source: VWO Blog
Source: Medium Article

3. MAB vs A/B Test
How ARE THEY DIFFERENT? PROS-CONS
Source: VWO Blog
Source: VWO Blog

4. BUSINESS USE-cases
Source: Vector Stock
Short-lived CAMPAIGNS
Multiple VARIATIONS
Source: Bob WP
Just saving Conversions
Source: Search Engine Journal

5. Core Components of VWO’s Bandit Algorithm
● Weight Initialization
● Weight Updation
● Traffic Split Computation
● Add exploration factor
Refer - Understanding the Working of Multi Armed Bandit in VWO to understand
mathematics in more details.

6. Weight Initialization
Content Weights
Individual performance of each variant
Content Interaction Weights(only MVT)
Paired performance of each variant
All weights are a distribution

7. Weight Updation
We use the Assumed Density Filtering algorithm
to model a layout’s conversion rate/revenue and
the message passing algorithm of Bayesian factor
graphs to update its corresponding weight
distributions.
This approach has helped us model conversion
rate/average revenue in MAB analytically,
helping us build a scalable solution for our
customers.
Message
Forward-Pass
Message
Backward-Pass

8. Thompson SAmpling(1993)
● Less trials means more uncertainty in
estimates. Spread/variance captures
uncertainty: enables Exploration.
● With more trials posteriors concentrate
on true parameter: enables Exploitation
For any system to sustain itself and adapt itself to the changing environment, it needs to explore while
exploiting constantly.

9. Traffic - Split Computation
1. Compute layout’s score -
○ Obtain a sample from weight
distributions corresponding to a
layout.
○ Add scores of all sampled weights.
2. Do step 1 for each layout.
3. Find the layout with the maximum score.
4. Do steps 1-3 several times, and we’ll obtain
a winning proportion of each layout which
will be the traffic split obtained from the
algorithm.

10. Add exploration factor
With thompson sampling after obtaining many data points, learning would come to a halt, and
the model will run in full exploitation mode. We use epsilon-greedy and thompson sampling to
determine traffic split to avoid convergence of a model to a single variation.
Therefore, after obtaining the traffic split from thompson sampling, we adjust the traffic split by
considering a fixed epsilon factor for exploration.

11. Performing statistical analysis in MAB Report
To ensure an equal proportion of visitors for
computing statistical reports-
● We take the minimum of the traffic
proportion obtained from MAB and use
it as a probability of a visitor to be
considered for statistical analysis.
● We mark the visitor based on the result of
a bernoulli trial.
So while performing statistical analysis, we
consider only marked visitors.

12. PROS/CONS OF OUR ALGORITHM
MAX-OUT AT
%

13. Revisiting the benefits and the tradeoffs

14. Questions?