Content-based Recommender Systems

jake@ twitter.com @pbraneUser Interest Modeling, Twitter Inc.

Apache Mahout PMC(previously: LinkedIn, a bunch of tiny startups)

Overview

• Collaborative Filtering == RecSys?

• User/Item content and/or metadata

• RecSys training w/ user/item features

• Advantages / Disadvantages

• Historical production examples from Twitter and LinkedIn

Recommender System

“traditional” RecSys

Aside: “users” don’t have to be users

• At LinkedIn, the Recommender Systems team built a general-purpose entity-to-entity RecSys: Product [user, item]

• TalentMatch [job posting, user-profile]

• GroupsYouMayLike [user, group]

• {Jobs for your group} [group, job-posting]

• AdsYouMayBeInterestedIn [user, ad]

Anmol Bhasin, Monica Rogati (now VP of Data at Jawbone), and myself built...So how do recommender systems work?next page: “an artists depiction of collaborative filtering”

Collaborative Filtering

CF is Generic!

• users / items reduced to GUIDs, could be anything

• large body of acad. work on techniques:

• SVD, ALS + other matrix factorizations

• stacked RBM, etc.

• General purpose OSS CF recommender:

• Apache Mahout (http://mahout.apache.org)

What about Domain Specific Knowledge?• Items are more than just GUIDs

• Users are more than just account names

• Perhaps, they are both much more:

• user profile text on Facebook / LinkedIn

• webdoc content + metadata

• movie genres, directors, description

• ad landing page content

could be derived data: at Twitter, every piece of text content passing through the system gets classified into topical categories, and users get classified according to their topical interests and things they’re “known for”

User/Item Features

• each user has a feature-vector

• each item has a feature-vector

• dimensionalities may (will!) differ

• collectively, we thus have MOAR Matrices!

next page: more art!

Feature Matrices

we could decompose this resultant user/item-feature matrix...

slight misrepresentation: user-features along rows of first matrix, columns are user-idsnote: the “multiplication” here could be actual matrix mult, OR maybe a more bayesian / statistical form: p(user|user-feature), p(item|user), p(item-feature|item) -> p(positive engagement | user-features, item-features). Full joint distribution ->HARD. Naive Bayes? or...

Train a ranker/classifier

• take a column of user-feature matrix:

• row of item-feature matrix:

• embed in

• train classifier to predict ratings given

go back and forth on this page to the previous onenext page is some notes about this

Training, cont.

• note: no need for any relationship between features

• if you apply a discretization technique, don’t even need to care about correlation between +/- values and “goodness/badness”

Classifier/Ranker RecSys HOWTO

• incoming preferences are triples of (user-feature vector, item-feature vector, preference-value)

• train classifier (online if desired!), and

• trained classifier spits out predicted rating given user/item pairs

• note: may require some item preselection

predicted ratings may not be what you want, it may be a Learn To Rank setup

Variations

• What if your features have some structure?

Structured data• Item = { field1, field2, field3, ... }

• User = { fieldA, fieldB, ... }

• field1: tf-idf-weighted “position description”

• field2 : standardized categorical job title

• field3 : #years experience

• fieldA : tf-idf “job requirements”

• fieldB : #years of experience required

note: this is TalentMatch: here “items” are LI profiles, and “users” are job postings

Pairwise-field similarity

• Some fields are naturally comparable to others, can compute vector cosine, jaccard, etc.

• Others have a business-specific similarity f(#years experience - required experience)

• Each set of field pairs generates an untrained weight

Train a low-dimensional classifier/ranker

• take these O(|item_fields| x |user_fields|) weights and feed into the training of a ranker.

• given low number of features, very interpretable

interpretation: p(user is good for job) = w_jobtitle+headline * sim(jobtitle, headline) + w_jobdesc+headline * sim(jobdesc, headline) + w_jobdesc+currentdesc * sim(jobdesc, currentdesc) + ...

Content-based RecSys: Pros

• Fixes cold-start problem

• Scales fantastically

• Flexible: can Learn To Rank using LR, SVM, GBDT, whatever

other content approach to cold-start: unsupervised similarity to engaged-with items/users + CFscales: use as much data as your classifier/ranker needs to converge well. once trained, can often be a very low latency method of generating item scores. Many classifiers are extremely insensitive to #features input

Content-based RecSys:Cons

• Not always very general (although: pairwise crossed features are pretty general)

• Features may be too coarse

• Feature selection may be difficult

• Low-latency from large item sets is hard

• Underweights popularity, similarity to known good items

for item selection: clustering can’t always work very well, if using crossed features, but sometimes tricks like LSH can help

Hybrid Models

• Classifiers/Regression models yield scores

• can combine this score with preferences from CF

• alternately, generate top-K items via CF, rerank with your content-based ranker (using CF rank as another feature)

Examples

• LinkedIn’s original (2010) generic entity-to-entity RecSys was primarily content-based

• Twitter’s #discover product is a hybrid recommender with content, social, and CF features

Note: PYMK is not primarily content basedAlso: personalized search is naturally a hybrid content-based recommender

Conclusion

• Free paper title: “On the unreasonable effectiveness of CF on the consumer web”

• But if you do know features about users/items: learn to rank using them!

• This is more common than you might think, in industry. But everyone’s got different domain-specific features, so less research about it

CF works absurdly well, given how little it knows about the items it’s recommendingRiff on Hardy’s “On the unreasonable effectiveness of mathematics in the physical sciences...”

Questions?

jake@twitter.com@pbrane

LinkedIn/G+ : jakemannix