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Case Study: Marktplaats.nl

@schuilr 2

Marktplaats.nl

•  Largest classifieds site in the Netherlands

•  One of the most visited websites in NL

•  Founded in 1999, acquired by eBay in 2004

•  Now headquarters to eBay Classifieds Group: 12 brands in 17 countries

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Facts & Figures

•  1.3 million visitors / day–  desktop: 34%, mobile: 49%, tablet: 18%

•  9 million live listings–  350,000 new items / day

•  6 million unique search requests / day–  70 searches per second (average)

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Data & Trends @ Marktplaats

Seasonal trends

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Vraa

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Week

skibroek

ski

skipak

snowboard

Winter sports!

Seasonal trends

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Camping!

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Week

caravans

campers

vouwwagen

Seasonal trends

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Week

sinterklaas

kerst

Saint Nicolas & Christmas!

Weather, temperature, etc.

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Tempe

ratuur)

Vraag)

Week)

vliegengordijn"

Temperatuur"

Fly curtains!

Weather, temperature, etc.

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Heaters!0"

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Tempe

ratuur)

Vraag)

Week)

kachel"

Temperatuur"

Reversed

Special events

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Week

oranje

Orange (“oranje”)!

World Cup Football

King’s Day

During a football game

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&

Last&Friday& This&Friday&

Break

Kick-off 1 - 0 1 - 1 1 - 2 1 - 3 1 - 4 1 - 5 End

“Juichpakken”

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Vraag%

Week%

roy$donders$

juichpak$

Exploiting trends

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“Nieuw & populair”

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•  “Nieuw & populair” = trending products

•  Pay-per-click advertising model

•  Advertisers bid for clicks, similar to Google Adwords

•  Metric to optimize: �Revenue Per Mille (RPM) = CTR * bid * 1,000

First (minimal) version

•  Find top 100 “trending” keywords using Spark•  Randomly pick one of those keywords•  Display top 4 results for the selected keyword

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Can we do better?

•  CTR and bid varies per keyword. Random selection gives average performance.

•  Doesn’t consider the user’s personal preferences

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GLOBAL OPTIMIZATION PART I

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One armed bandit = slot machine�

Problem:�How to pick between slot machines �

so that you maximize profit?

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Exploration – Exploitation

•  Explore (learn)"Try out different candidates to learn how they perform over time

•  Exploit (earn)"Take advantage of what you’ve learned to maximize payoff (your current best guess)

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Many different approaches

•  Epsilon First•  Epsilon Greedy•  Upper Confidence Bound•  Thompson Sampling•  LinUCB

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Epsilon First

Time

Random

Learn: collect data for each

candidate

( split testing, A/B testing )

Best

Earn: show the best

performer

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Epsilon First •  Simple and intuitive•  Lots of tools available (VWO, Optimizely, …)�

•  Average reward until exploration is finished•  What if the best candidate is no longer the best?

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Epsilon Greedy

Best (90%)

Time

Random (10%)

Continuous exploration

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Epsilon Greedy •  Very simple to implement and surprisingly effective•  Can deal with nonstationary problems

•  How to determine the optimal value for ε?

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Upper Confidence Bound Basic idea:•  Calculate mean and a measure of uncertainty

(variance) for each candidate•  Pick current best performer based on mean +

uncertainty bonus

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Measuring uncertainty

Observed mean: 0.50

95% certain that true mean ≤ 0.76

Uncertainty bonus: 0.26

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More data = less uncertainty

95% certain that true mean ≤ 0.63

Uncertainty bonus: 0.13

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Mean + uncertainty bonus

Upper Confidence Bound

A

B

C

Es)matedreward

Pick “A”!

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Upper Confidence Bound •  Selecting “A” reduces uncertainty•  Candidate “C” now has the highest score

A

B

C

Es)matedreward

Pick “C”!

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Upper Confidence Bound

•  Uses variance measure to automatically balance exploration with exploitation�

•  Deterministic; requires online learning (not suited for small-batch mode)

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Thompson Sampling Basic idea:•  The number of pulls for a given lever should match

its actual probability of being the optimal lever�

•  Sample from the posterior for the mean of each lever:�

p(λ|X) = Gamma(conv + prior_conv, impr + prior_impr)

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Few conversions Candidate Conversions Impressions Chance of being

winner

A (3.9%) 11 282 42%

B (3.3%) 2 61 39%

C (2.8%) 4 143 19%

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More conversions Candidate Conversions Impressions Chance of being

winner

A (3.9%) 93 2,382 82%

B (3.3%) 66 2,011 13%

C (2.8%) 31 1,093 5%

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Many conversions Candidate Conversions Impressions Chance of being

winner

A (3.9%) 892 22,882 97%

B (3.3%) 174 5,261 2%

C (2.8%) 66 2,343 1%

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Lots of conversions Candidate Conversions Impressions Chance of being

winner

A (3.9%) 5,621 144,132 > 99%

B (3.3%) 256 7,761 < 1%

C (2.8%) 101 3,593 < 1%

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Thompson Sampling

•  Weighted random sampling•  Works well in small-batch mode�

•  Doesn’t consider context (e.g. user’s personal preferences)

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PERSONALIZATION PART II

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LinUCB Basic idea:•  Define a “context” of information of the user•  Fit a per-candidate logistic regression model•  Applies the concept of Upper Confidence Bound

(UCB)–  mean + uncertainty bonus

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Context •  Gender•  Recently viewed categories•  Current date•  Weather forecast•  …

Principal Component Analysis (PCA) to reduce sparseness and computation complexity

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LinUCB Mean + uncertainty bonus:

μα(t) + σα(t)

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Pruning

•  Periodically remove weakest performers•  Replace with new, unexplored “trending keywords”•  Rinse and repeat

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Results

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Random Optimized

× 2.8!

Endless possibilities

•  News homepage•  Online advertising•  Deciding which thumbnail to show on the SERP•  Etc, etc ...

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Reading List “Bandit Algorithms for Website Optimization”�http://bit.ly/bandits-book

“Reinforcement Learning”�http://bit.ly/rl-book

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@SCHUILR"LINKEDIN.COM/IN/ROBINSCHUIL

Дякую

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References •  https://en.wikipedia.org/wiki/Multi-armed_bandit•  http://shop.oreilly.com/product/0636920027393.do•  https://webdocs.cs.ualberta.ca/~sutton/book/the-book.html•  http://www.slideshare.net/chucheng/efficient-approximate-thompson-sampling-for-search-query-recommendation•  http://www.slideshare.net/iliasfl/multiarmed-bandits-intro-examples-and-tricks•  http://www.slideshare.net/mgershoff/conductrics-bandit-basicsemetrics1016•  http://www.slideshare.net/MarkusOjala1/multi-armed-bandits-and-optimized-online-marketing-54679491

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