Recommender Systems based on Linked Open Data

@cataldomusto
Recommender Systems
based on Linked Open Data
CATALDO MUSTO
UNIVERSITÀ DEGLI STUDI DI BARI ‘ALDO MORO’ - ITALY
Research Workshop of the
Israel Science Foundation
on User Modeling and
Recommender Systems
Haifa, Israel
July 19, 2017
cataldo.musto@uniba.it

What are we going to talk about?
Cataldo Musto. Recommender Systems based on Linked Open Data.
Research Workshop of the Israel Science Foundation on User Modeling and Recommender Systems. Haifa, Israel. July 19, 2017

What are we going to talk about?
Semantics
(in Recommender Systems)

The genesis
Semantic Web
[*] Berners-Lee, Tim; James Hendler; Ora Lassila
"The Semantic Web". Scientific American Magazine, 2001
“The Semantic Web provides a common framework that
allows data to be shared and reused across application
enterprise, and community boundaries” [*]

The genesis
Semantic Web
(Do we succed?)

The genesis
Semantic Web
Linked Open Data Project
Goal: to make structured and interconnected the
whole DATA available on the Web [^].
[^] C. Bizer, T. Heath e T. Berners-Lee,
Linked Data—The Story So Far .International Journal on
Semantic Web and Information Systems, 5(3), 2009

Linked Open Data
What is it?
(basic Italian gesture)

Linked Open Data
What is it?
(basic Italian gesture)
Linked Open Data is a
methodology
to publish, share and link
structured data on the Web

Linked Open Data: cornerstones
1. Use of RDF to model the information and make data
publicly available

publicly available
subject object
predicate
(this is called RDF triple)

publicly available
Keanu Reeves The Matrix
acted

publicly available
Keanu Reeves The Matrix
acted
URI URI / Literal

publicly available
dbr:Keanu_Reeves dbr:The_Matrix
acted
URI URI

publicly available
dbr:Keanu_Reeves 1964
birthYear
URI Literal

publicly available
2. Re-Use existing resources and properties
in order to make the data inter-connected
acted

publicly available
2. Re-Use existing resources and properties
in order to make the data inter-connected
dbo:starring
acted

Linked Open Data
We only use a small subset
of the ‘Semantic web cake’

Linked Open Data
We only use a small subset
of the ‘Semantic web cake’
We use RDF to model our
data and we use SPARQL
as query language
to gather data

Linked Open Data
Do we succeed?

Linked Open Data cloud
This is the Linked Open Data cloud

It is a (huge) set of interconnected
semantic datasets
Each bubble is a dataset!

semantic datasets
How many datasets we have?
149 billions triples
and 9,960 datasets
(source: http://stats.lod2.eu)

Research Workshop of the Israel Science Foundation on User Modeling and Recommender Systems Haifa, Israel. July 19, 2017
Datasets cover many domains
semantic datasets

The core of the
is DBpedia (http://www.dbpedia.org)
RDF mapping
of Wikipedia

DBpedia
Wikipedia
Unstructured Content

DBpedia
Wikipedia
Unstructured Content
DBpedia
Structured Data

DBpedia
All the
information
available in
Wikipedia
is modeled
in RDF

In a nutshell
We have interesting
features coming from
Wikipedia (and other
sources) and the
advantage of formal
semantics defined in RDF

In a nutshell
We have semantics
without the need of
building and manually
populating an ontology
We have interesting
features coming from
Wikipedia (and other
sources) and the
advantage of formal
semantics defined in RDF

One step back…

One step back…
SPARQL comes into play!

SPARQL
An example of SPARQL query
[…]
SELECT DISTINCT ?city ?name
WHERE {
?city dct:subject dbc:Cities_in_Israel .
?city rdfs:label ?name .
?city dbo:populationTotal ?population .
FILTER (?population > 100000) .
FILTER (lang(?name) = 'en')
}

SPARQL
An example of SPARQL query
[…]
WHERE {
}
Returns big
cities in Israel
(more than
100,000
people)

SPARQL
[…]
WHERE {
}
Returns big
cities in Israel
(more than
100,000
people)
How do we exploit SPARQL ?

SPARQL
[…]
WHERE {
}
Returns big
cities in Israel
(more than
100,000
people)
Key concept: mapping

SPARQL
?
Given an item, we need to find
an ‘entry point’ to the LOD cloud

SPARQL
SELECT DISTINCT ?uri, ?title
WHERE {
?uri rdf:type dbpedia-owl:Film.
?uri rdfs:label ?title.
FILTER langMatches(lang(?title), "EN") .
FILTER regex(?title, "matrix", "i")
}
We can run a SPARQL query to find the
corresponding URI for the resource

SPARQL
SELECT DISTINCT ?uri, ?title
WHERE {
?uri rdf:type dbpedia-owl:Film.
?uri rdfs:label ?title.
FILTER langMatches(lang(?title), "EN") .
FILTER regex(?title, "matrix", "i")
}
Once we have a mapping,
properties can be extracted
dbr:The_Matrix

LOD-aware data model
Once we have a mapping, properties can be extracted

Research Question
How can we use Linked Open Data for Recommender Systems?

Motivations: Limited Content Analysis
In some scenarios,
we don’t have
enough features to
feed our
recommendation
models.
LOD cloud
can be helpful

Motivations: Limited Content Analysis
Several very
fine-grained
and interesting
features can be
easily injected
by querying
DBpedia

Motivations: Graph-based Data Model
Basic
Graph-based
Data Model
Only collaborative
connections are
modeled
User-2
User-1

Extended
Graph-based
Data Model
Richer
representation
based on properties
gathered from the
LOD cloud
User-2
User-1

User-2
Extended
Graph-based
Data Model
New and
unexpected
connection may
lead to more
surprising
recommendations
User-1

Recommender Systems based on Linked Open Data

Recommender Systems based on Linked Open Data
1.Approaches based on
Vector Space Models
Graph-based Models
Machine Learning
techniques

LOD-based RecSys: approaches based on VSM
In this case, LOD are typically used to cope with the limited content analysis problem

First, a data source
is needed

Thanks to the LOD, we can obtain a richer vector-space item representation

similarity between items

similarity between items
Can we think about more complex models?

In DBpedia each item is modeled on the
ground of several facets
Tommaso Di Noia, Roberto Mirizzi, Vito Claudio Ostuni, Davide Romito, Markus Zanker.
Linked Open Data to support Content-based Recommender Systems. 8th International
Conference on Semantic Systems (I-SEMANTICS) - 2012 (Best Paper Award)

In DBpedia each item is modeled on the
ground of several facets
Each facet is modeled as a
slice of a tensor.
Each slice encodes the features
describing that particular facet.

Similarity between the items as linear combination
of the similarity among each DBpedia facet

ǁ𝑟 𝑢, 𝑥𝑗 =
σ 𝑥 𝑖∈𝑃𝑟𝑜𝑓𝑖𝑙𝑒(𝑢) 𝑟 𝑢, 𝑥𝑖 ⋅
σ 𝑝∈𝑃 𝛼 𝑝 ⋅ 𝑠𝑖𝑚 𝑝(𝑥𝑖, 𝑥𝑗)
|𝑃|
|𝑝𝑟𝑜𝑓𝑖𝑙𝑒(𝑢)|
Predict the rating using a nearest neighbor Classifier
wherein the similarity measure is a linear combination
of local property similarities

best solution achieved
with
subject+broader+genres
too many broaders
introduce noise

LOD-based RecSys: graph-based data models
(bipartite graph)

Basic graph-based data models only
encode collaborative data points
We can extend such data model by
introducing features gathered from
the LOD cloud
(bipartite graph)

Mapping

Once the mapping is
completed, we can
extend the graph by
injecting the
descriptive properties
gathered from the
LOD cloud
Many new interesting
connections are
modeled in the graph

The process can be
repeated in order to
expand the graph with
broader properties
(2-hop graph)

The process can be
broader properties
(2-hop graph)
Up to n expansion
steps (n-hop graph)

The process can be
broader properties
(2-hop graph)
Up to n expansion
steps (n-hop graph)
How do we get the
recommendations?

Recommendations
obtained by mining
the graph

Recommendations
obtained by mining
the graph
Identification of the
most relevant (target)
nodes, according to
the recommendation
scenario

Recommendations
obtained by mining
the graph
Identification of the
most relevant (target)
nodes, according to
the recommendation
scenario
PageRank
Spreading Activation
Personalized PageRank
…

Typically, the
relevance score for all
the item nodes is
calculated and the
top-N are returned as
recommendations

Typically, the
relevance score for all
the item nodes is
calculated and the
top-N are returned as
recommendations
Research Question
Is this a good
recommendation model?
How do LOD impact on
the accuracy?

Recent work [*]
Task: top-N recommendation
Expansion: 1-hop, all the
properties were injected
Recommendation algorithm:
PageRank with Priors
Settings: Hot Start, Cold Start
Topologies: NoLOD , LOD
[*] C. Musto, P. Basile, P. Lops, M. de Gemmis, G. Semeraro: Introducing linked open data
in graph-based recommender systems. Inf. Process. Manage. 53(2): 405-435 (2017)

55,02
46,35
55,04
47,55
42
44
46
48
50
52
54
56
DBbook Last.fm
No LOD vs. LOD – Hot Start Scenario (F1@5)
No-LOD LOD
C. Musto, P. Basile, P. Lops, M. de Gemmis, G. Semeraro: Introducing linked open
data in graph-based recommender systems. Inf. Process. Manage. 53(2): 405-435 (2017)

53,16
42,57
52,94
45,56
0
10
20
30
40
50
60
DBbook Last.fm
No LOD vs. LOD – Cold Start Scenario (F1@5)
No-LOD LOD

Side Research Questions
Are all the properties equally
important?
Is it possible to identify the
most relevant properties?
X
X

Side Research Questions
Are all the properties equally
important?
Is it possible to identify the
most relevant properties?
How does this affect the
overall accuracy?
X
X

How can we choose the most
promising properties?

manual selection
domain-specific properties
most frequent properties
…
automatic selection
more difficult to implement

manual selection
domain-specific properties
most frequent properties
…
automatic selection
more difficult to implement
We compared seven
different techniques for
automatic features
selection
PageRank mRMR
Chi-Square PCA
Gain Ratio SVM
Information Gain

MovieLens data / F1@10
baseline C. Musto, P. Basile, P. Lops, M. de Gemmis, G. Semeraro: Introducing linked open

DBbook data / F1@10
baseline C. Musto, P. Basile, P. Lops, M. de Gemmis, G. Semeraro: Introducing linked open

DBbook data / F1@10
Features Selection techniques significantly improve the
predictive accuracy of graph-based recommendation models

DBbook data / F1@10
What about state-of-the-art techniques?

Comparison to state of the art
MovieLens
DBbook

LOD-based RecSys: machine learning techniques
new features describing the item
can be inferred by mining the
structure of the tripartite
graph

Average Neighbor degree
Degree Centrality
Node redundancy
Clustering coefficient
new features describing the item
can be inferred by mining the
structure of the tripartite
graph

Research Question: what is the impact of such
features on the overall performance of the
recommendation framework?
C. Musto, G. Semeraro, M. de Gemmis, P. Lops. A Hybrid Recommendation Framework Exploiting Linked Open Data
and Graph-based Features. 25° Int. Conf. on User Modeling, Adaptation and Personalization (UMAP 2017)

Insight: to build a hybrid classification framework
exploiting LOD-based and graph-based features
Research Question: what is the impact of such
features on the overall performance of the
recommendation framework?

Item Representation & Methodology
We first model basic features

Then we introduce extended features based on the LOD cloud

We use them to feed a hybrid classification framework

Results
Collaborative and
popularity-based
features get the
best results

Results
LOD-based and
graph-based
features got
the best results

LOD-based RecSys
Take Home Messages
1. Linked Open Data represent a huge data silos, which is freely available
2. They can easily let overcome the limited content analysis problem
3. They can enrich graph-based data model with interesting data points
4. They can feed machine learning models with new and relevant features
5. They improve the accuracy of recommender systems

LOD-based RecSys
Take Home Messages
Future Trends
• Linked Open Data && (Graph Embeddings || Word Embeddings)
• Linked Open Data && Different Metrics (Serendipity, Novelty, etc.)
1. Linked Open Data represent a huge data silos, which is freely available
2. They can easily let overcome the limited content analysis problem
3. They can enrich graph-based data model with interesting data points
4. They can feed machine learning models with new and relevant features
5. They improve the accuracy of recommender systems

Questions?
Research Workshop of the Israel Science Foundation on User Modeling and Recommender Systems Haifa, Israel. July 19, 2017
cataldo.musto@uniba.it
@cataldomusto

Recommender Systems based on Linked Open Data

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