1) The document discusses recommender system algorithms and architecture. It covers common recommendation techniques like collaborative filtering, content-based filtering, and graph-based recommendations. 2) It also discusses challenges like cold starts for new users and items. For new users, it recommends using demographic data or initial feedback to understand interests. For new items, it suggests using content information. 3) The document proposes a feature-based recommendation framework that connects users, items, and their features to address challenges like cold starts and providing explanations. This framework represents users and items in a feature space to make recommendations.

1. Recommender System:
Algorithms & Architecture
xiangliang@hulu.com

2. Outline
Problem
Data
•
Algorithms
•
Cold start
•
Architecture
•
•
Recommender
System

3. Problem
Recommend items to users to make user, content
partner, websites happy!

4. Data
• User behaviors data
Page view All user Very Large
Behavior User Size
Watch video All user Large
Favorite Register user Middle
Vote Register user Middle
Add to playlist Register user Small
Facebook like Register user Small
Share Register user Small
Review Register user Small

5. Data
• Which data is most
important Page view All user Very Large
Behavior User Size
– Main behavior in the
Watch video All user Large
website
Favorite Register user Middle
Vote Register user Middle
– All user can have such Add to playlist Register user Small
behavior
Facebook like Register user Small
– Cost
Share Register user Small
Review Register user Small
– Reflect user interests on
items

6. Data
• Data Structure
– User ID
– Item ID
– Behavior Type
– Behavior Content
– Context
• Timestamp
• Location
• Mood
Sheldon watch Star Trek with his friends at home

7. Algorithms
Recommender
System Method
Collaborative Content Social
……
Filtering Filtering Filtering
Latent Factor
Graph-based ……
Model
Neighborhood
……
-based
User-based Item-based ……

8. Neighborhood-based
• User-based
– Digg
• Item-based
– Amazon, Netflix, YouTube, Hulu, …

9. User-based
• Algorithm
– For user u, find a set of users S(u) have similar
preference as u.
– Recommend popular items among users in S(u)
to user u.

10. User-based CF
pui = ∑
v∈S ( u , K ) ∩ N ( i )
wuv rvi
N (u ) ∩ N (v)
w uv =
N (u ) ∪ N (v)

11. Item-based
• Algorithm
– For user u, get items set N(u) this user like
before.
– Recommend items which are similar to many
items in N(u) to user u.

12. Item-based CF
pui = ∑
j∈S ( i , K ) ∩ N ( u )
w ji ruj
N (i ) ∩ N ( j )
w ij =
N (i ) ∪ N ( j )

13. Item-based CF
Why not use w ij = ?
N (i ) ∩ N ( j )
N (i )

14. Neighborhood-based
• User-based vs. Item-based
User-based Item-based
Scalability Bad when user size is Bad when item size is
large large
Explanation Bad Good
Novelty Bad Good
Coverage Bad Good
Cold start Bad for new users Bad for new items
Performance Need to get many Only need to get
users history current user’s history

15. References
• Amazon.com Recommendations item-to-
item Collaborative Filtering.
• Empirical Analysis of Predictive Algorithms
for Collaborative Filtering.

16. Graph-based
• Users’ behaviors on items can be
represented by bi-part graph.
A 1 A 1 A 1 A 1
B 2 B 2 B 2 B 2
C 3 C 3 C 3 C 3
D 4 D 4 D 4 D 4

17. Graph-based
• Two nodes will have high relevance if
– There are many paths in graph between two
nodes.
– Most of paths between two nodes is short.
– Most paths do not go through nodes with high
out-degree.

18. Graph-based
• Advantage
– Heterogeneous data A 1
• Multiple user behaviors
• Social Network
B 2
• Context (Time, Location) C 3
• Disadvantage D 4
– Statistical-based
– High cost for long path

19. References
• A Graph-based Recommender System for
Digital Library.
• Random-walk computation of similarities
between nodes of a graph with application
to collaborative recommendation.

20. Latent Factor Model
• Users and items are connect by latent
features.
A 1
a
B 2
b
C 3
c
D 4

21. Latent Factor Model
rui = ∑ puk qik
ˆ
k
Science Fiction 0.5 Science Fiction 0.9
Universe 0.9 Universe 0.9
Physical 0.8 Physical 0.5
Space Travel 0.8 Space Travel 0.7
Animation 0.3 Animation 0.1
Romance 0.0 Romance 0.0

22. Latent Factor Model
• How to get p, q?
min ∑ (rui − ∑ puk qik ) + λ ( pu + qi )
2 2 2
( u ,i ) k
= α (eui qik − λ puk )
puk +
= α (eui puk − λ qik )
qik +

23. Latent Factor Model
• How to define rui
– Rating prediction
– Top-N recommendation
• Implicit feedback data: only have positive samples
and missing values, how to select negative samples?

24. Latent Factor Model
1 (Sci-fi) 2 (Crime) 3 (Family) 4 (Horror)
The Blair Witch
The invisible Man Jaws 101 Dalmatians
Project
Frankenstein
Back to the
Meets the Wolf Lethal Weapon Pacific Heights
Future
Man
Godzilla Total Recall Groundhog Day Stir of Echoes
Star Wars VI Reservoir Dogs Tarzan Dead Calm
The Terminator Donnie Brasco The Aristocats Phantasm
The Jungle Book
Alien The Fugitive Sleepy Hollow
2
Alien 2 La shou Shen tan Antz The Faculty

25. Latent Factor Model
• Advantage
– High accuracy in rating prediction
– Auto group items
– Scalability is good
– Learning-based
• Disadvantage
– Incremental updating
– Real-time
– Explanation

26. References
• http://www.informatik.uni-
trier.de/~ley/db/indices/a-
tree/k/Koren:Yehuda.html

27. Cold Start
• Problems
– User cold start : new users
– Item cold start : new items
– System cold start : new systems

28. User Cold Start
• How to recommend items to new users?
– Non-personalization recommendation
• Most popular items
• Highly Rated items
– Using user register profile (Age, Gender, …)

29. User Cold Start
• Example: Gender and TV shows
Data comes from IMDB : http://www.imdb.com/title/tt0412142/ratings

30. User Cold Start
Male
Age : 20-30
Theoretical physicist
Doctor
American
Irreligious

31. How to get user interest quickly
• When new user comes, his feedback on
what items can help us better understand
his interest?
– Not very popular
– Can represent a group of items
– Users who like this item have different
preference with users who dislike this item

32. Item Cold Start
• How to recommend new items to user?
– Do not recommend
How to recommend news??

33. Item Cold Start
• How to recommend new items to user?
– Using content information
Machine
Data Mining Recommendation
Learning

34. System Cold Start
• How to design recommender system when
there is no user?
– Pandora : Music Genome Project
– Jinni : Movie Genome Project

35. Architecture
• Feature-based recommendation framework:
A 1
a
B 2
b
C 3
c
D 4
User Feature Item

36. Architecture
Male
Scientist
Physics

37. Architecture
• Advantage:
– Heterogeneous data
– Reasonable Explanation
• Disadvantage:
– Do not support user-based methods

38. Open Questions
• How to weight multiple behaviors?
• How to improve diversity, novelty?
• How to build feedback loop?

39. Thanks!