recommendation systemssrijan/teaching/cse6240/... · •recommendation systems are ubiquitous...

Srijan Kumar, Georgia Tech, CSE6240 Spring 2020: Web Search and Text Mining 1

CSE 6240: Web Search and Text Mining. Spring 2020

Recommendation Systems

Prof. Srijan Kumarhttp://cc.gatech.edu/~srijan


Today’s Lecture• Introduction• Content-based Models• Collaborative Filtering• Latent Factor Models • Deep Learning Models

Reference material: http://mmds.org/


RecSys Drives The World

• Recommendation systems are ubiquitous– E-commerce: Products to buyers – Job applications: Jobs to applicants, Applicants

to recruiters – Dating: Partners– Services: Doctors, lawyers, restaurants– Social networks: News that you read, posts that

you see• Most of what you do is powered or

influenced by some recommender system!


Types of Recommendations• Editorial and hand curated

– List of favorites– Lists of “essential” items

• Simple aggregates– Top 10, Most Popular, Recent Uploads

• Personalized to individual users– Amazon, Netflix, …– Main focus of this class


General Setting • Input:

– U = Set of users – I = Set of items – R = Set of observed

ratings between subset of users and subset of items

• Output: – Predict ratings for any

(user, item) pair

Avatar LOTR Matrix Pirates

Alice 1 5

Bob 3 1

Carol 1

David 4


Recommendations: Key Challenges

• (1) Gathering “known” ratings for matrix– How to collect the data in the utility matrix

• (2) Extrapolate unknown ratings from the known ones– Mainly interested in high unknown ratings

• We are not interested in knowing what you don’t like but what you like

• (3) Evaluating extrapolation methods– How to measure success/performance of

recommendation methods


Challenge 1: Gathering Ratings• Explicit

– Ask people to rate items– Doesn’t work well in practice – people

can’t be bothered

• Implicit– Learn ratings from user actions

• E.g., purchase implies high rating– What about low ratings?


Challenge 2: Extrapolating Utilities• Key problem: Utility matrix U is sparse

– Most people have not rated most items– Cold start:

• New items have no ratings• New users have no history

• Four approaches:– 1) Content-based– 2) Collaborative– 3) Latent factor based– 4) Deep learning based

Our focus


Challenge 3: Evaluations • Metrics to evaluate recommendations and

ranking lists • Online setting: Purchase rate, Click-through

rate • Offline setting: which metrics to use?

• How do we evaluate recommender systems? – Need evaluation setting and metric

Srijan Kumar, Georgia Tech, CSE6240 Spring 2020: Web Search and Text Mining

Evaluation Setting

1 3 4

3 5 5

4 5 5

3

3

2 2 2

5

2 1 1

3 3

1

movies

users

14

• Input: rating matrix

Srijan Kumar, Georgia Tech, CSE6240 Spring 2020: Web Search and Text Mining

Evaluation Setting

1 3 4

3 5 5

4 5 5

3

3

2 ? ?

?

2 1 ?

3 ?

1

Test Data Set

users

movies

11

•Hide some known ratings and try to predict it correctly


Evaluation Metrics

• Compare predictions with known ratings– Root-mean-square error (RMSE)

• ∑ 𝑟#$ − 𝑟#$∗'�

#$�

where 𝒓𝒙𝒊 is predicted, 𝒓𝒙𝒊∗ is the true rating of x on i

– Precision at top 10: • % of correct item in top 10

– Rank Correlation: • Spearman’s correlation between system’s and user’s

complete rankings


Evaluation Metrics

• Another approach: 0/1 model– Coverage:

• Number of items/users for which system can make predictions

– Precision:• Accuracy of predictions

– Receiver operating characteristic (ROC)• Tradeoff curve between false positives and false

negatives


Problems with Evaluation Measures• Narrow focus on accuracy sometimes

misses the point– Prediction Diversity– Prediction Context– Order of predictions

• In practice, we care only to predict high ratings:– RMSE might penalize a method that does well

for high ratings and badly for others


RecSys: Approach Styles1. Content-based: which items have

properties similar to the items you already like?

2. Collaborative filtering: what do similar users like?

3. Latent factor based: learn latent representation of all users’ behavior

4. Deep learning based: combine everything together in a unified, deep framework


Recommendation Systems• Content-based Models• Collaborative Filtering• Latent Factor Models • Deep Learning Models


Content-based Recommendations• Main idea: Recommend items to customer x

similar to previous items rated highly by x

Example:• Movie recommendations

– Recommend movies with same actor(s), director, genre, …

• Websites, blogs, news– Recommend other sites with “similar” content


Plan of Action

likes

Item profiles

RedCircles

Triangles

User profile

match

recommendbuild


Item Profiles• For each item, create an item profile i

• Profile is a set (vector) of features– Movies: author, title, actor, director,…– Text: Set of “important” words in document

• How to pick important features?– Usual heuristic from text mining is TF-IDF

(Term frequency * Inverse Doc Frequency)• Feature = term• Item = Document


Recall: TF-IDF• fij = frequency of term (feature) i in doc (item) j

• ni = number of docs that mention term i• N = total number of docs

• TF-IDF score: wij = TFij × IDFi

• Doc profile = set of words with highest TF-IDF scores, together with their scores


User Profiles and Prediction

• User profile possibilities:– Weighted average of rated item profiles

• Prediction heuristic:– Given user profile x and item profile i,

estimate 𝑢(𝒙, 𝒊) = cos(𝒙, 𝒊) = 𝒙·𝒊| 𝒙 |⋅| 𝒊 |


Pros: Content-based Approach

• No need for data on other users– No cold-start or sparsity problems

• Able to recommend to users with unique tastes

• Able to recommend new & unpopular items– No first-rater problem

• Able to provide explanations– Can provide explanations of recommended

items by listing content-features that caused an item to be recommended


Cons: Content-based Approach

• Finding the appropriate features is hard– E.g., images, movies, music

• Recommendations for new users– How to build a user profile?

• Overspecialization– Never recommends items outside user’s

content profile– People might have multiple interests– Unable to exploit quality judgments of other

users


Recommendation Systems• Content-based• Collaborative Filtering• Latent Factor Models • Deep Learning Models


Collaborative Filtering• Harnessing quality judgments of other

users• Consider user x• Find set N of other

users whose ratings are “similar” to x’s ratings

• Estimate x’s ratings based on ratings of users in N

x

N


Finding “Similar” Users• How to find similarity between users? • rx = vector of user x’s ratings; ry = y’s ratings

• Metric 1: Jaccard similarity measure– Jaccard = (Number of common items rated by

both) / (Number of items rated by either)– Problem: Ignores the value of the rating

rx = [*, _, _, *, ***]ry = [*, _, **, **, _]


Finding “Similar” Users• Metric 2: Cosine similarity measure

– Treats rx and ry as vectors– sim(x, y) = cos(rx, ry) = 89⋅8:

||89||⋅||8:||

– Problem: Treats missing ratings as “negative”• Metric 3: Pearson correlation coefficient

– Find degree of similarity of ratings– Sxy = items rated by both users x and y– Problem: Ignores items rated by only one

rx, ry as vectors:rx = {1, 0, 0, 1, 3}ry = {1, 0, 2, 2, 0}

rx, ry = avg.rating of x, y

𝒔𝒊𝒎 𝒙, 𝒚 =∑ 𝒓𝒙𝒔 − 𝒓𝒙 𝒓𝒚𝒔 − 𝒓𝒚�𝒔∈𝑺𝒙𝒚

∑ 𝒓𝒙𝒔 − 𝒓𝒙 𝟐�𝒔∈𝑺𝒙𝒚

� ∑ 𝒓𝒚𝒔 − 𝒓𝒚𝟐�

𝒔∈𝑺𝒙𝒚�


Similarity Metric: Solution

• Intuitively we want: sim(A, B) > sim(A, C)• But, Jaccard similarity: 1/5 < 2/4• Cosine similarity: 0.386 > 0.322

– Problem: Considers missing ratings as “negative”– Solution: Normalize by subtracting the row mean.

Then calculate the cosine similarity.– Note: Cosine similarity is correlation when the

data is centered at 0


Similarity Metric: Solution

sim(A,B) vs sim(A,C):0.092 > -0.559

sim(A,B) vs sim(A,C):0.386 > 0.322

Subtract the row mean


Rating PredictionsFrom similarity metric to recommendations:• Let rx be the vector of user x’s ratings• Let N be the set of k users most similar to x

who have rated item i• Prediction for item s of user x:

– 𝑟#$ =AB∑ 𝑟C$�

C∈D

– 𝑟#$ =∑ E9:⋅8:F�:∈G∑ E9:�:∈G

Shorthand:𝑠#C = 𝑠𝑖𝑚 𝑥, 𝑦


Item-Item Collaborative Filtering

• So far: User-user collaborative filtering• Another view: Item-item

– For item i, find other similar items– Estimate rating for item i based on ratings for

similar items– Can use same similarity metrics and

prediction functions as in user-user model

åå

Î

Î×

=);(

);(

xiNj ij

xiNj xjijxi s

rsr

sij = similarity of items i and jrxj = rating of user u on item jN(i;x)= set items rated by x similar to i


Item-Item CF (|N|=2)

121110987654321

455311

3124452

534321423

245424

5224345

423316

users

mov

ies

unknown rating

rating between 1 to 5

estimate rating of movie 1 by user 5

Neighbor selection: Identify movies similar to movie 1, rated by user 5


Item-Item CF (|N|=2)users

mov

ies

1.00

-0.18

0.41

-0.10

-0.31

0.59

sim(1,m)

Use Pearson correlation as similarity: Subtract mean rating mi from each movie i. Mean m1 = (1+3+5+5+4)/5 = 3.6

row 1: [-2.6, 0, -0.6, 0, 0, 1.4, 0, 0, 1.4, 0, 0.4, 0]Then, compute cosine similarities between rows.

121110987654321

455311

3124452

534321423

245424

5224345

423316


Item-Item CF (|N|=2)users

mov

ies

1.00

-0.18

0.41

-0.10

-0.31

0.59

sim(1,m)121110987654321

455311

3124452

534321423

245424

5224345

423316

Predict by taking weighted average:

r1,5 = (0.41*2 + 0.59*3) / (0.41+0.59) = 2.6𝑟$# =

∑ 𝑠$M ⋅ 𝑟M#�M∈D($;#)

∑𝑠$M


CF: Extended Version• Define similarity sij of items i and j• Select k nearest neighbors N(i; x)

– Items most similar to i, that were rated by x• Estimate rating rxi as the weighted average:

baseline estimate for rxi

¡ μ =overallmeanmovierating¡ bx =ratingdeviationofuserx

=(avg.ratingofuserx) – μ¡ bi =ratingdeviationofmoviei

åå

Î

Î-×

+=);(

);()(

xiNj ij

xiNj xjxjijxixi s

brsbr

𝒃𝒙𝒊 = 𝝁 + 𝒃𝒙 + 𝒃𝒊


Item-Item vs. User-User¡ In practice, it has been observed that item-

item often works better than user-user¡ Why? Items are simpler, users have

multiple tastes


Pros of Collaborative Filtering

• Works for any kind of item– No feature selection needed

• Leverages other users’ behavior– Incorporates community actions


Cons of Collaborative Filtering

• Cold Start:– Need enough users in the system to find a match

• Sparsity:– The user/ratings matrix is sparse– Hard to find users that have rated the same items

• First rater: – Cannot recommend an item that has not been

previously rated. What about new items, esoteric items?

• Popularity bias: – Cannot recommend items to someone with

unique taste – Tends to recommend popular items


Collaborative Filtering: Complexity• Expensive step is finding k most similar

customers: O(|X|) • Too expensive to do at runtime

– Could pre-compute• Naïve pre-computation takes time O(k ·|X|)

– X … set of customers

• We already know how to do this!– Near-neighbor search in high dimensions (LSH)– Clustering– Dimensionality reduction


Hybrid Methods• Implement two or more different

recommenders and combine predictions– Perhaps using a linear model

• Add content-based methods to collaborative filtering– Item profiles for new item problem– Demographics to deal with new user problem


Recommendation Systems• Content-based• Collaborative Filtering• Latent Factor Models • Deep Learning Models

recommendation systemssrijan/teaching/cse6240/... · •recommendation systems are ubiquitous...

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