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<article xmlns:xlink="http://www.w3.org/1999/xlink">
  <front>
    <journal-meta />
    <article-meta>
      <article-id pub-id-type="doi">10.1016/j.knosys.2020.105618</article-id>
      <title-group>
        <article-title>Knowledge Graph Datasets for Recommendation</article-title>
      </title-group>
      <contrib-group>
        <contrib contrib-type="author">
          <string-name>Vincenzo Paparella</string-name>
          <xref ref-type="aff" rid="aff0">0</xref>
        </contrib>
        <contrib contrib-type="author">
          <string-name>Alberto Carlo Maria Mancino</string-name>
          <xref ref-type="aff" rid="aff0">0</xref>
        </contrib>
        <contrib contrib-type="author">
          <string-name>Antonio Ferrara</string-name>
          <xref ref-type="aff" rid="aff0">0</xref>
        </contrib>
        <contrib contrib-type="author">
          <string-name>Claudio Pomo</string-name>
          <xref ref-type="aff" rid="aff0">0</xref>
        </contrib>
        <contrib contrib-type="author">
          <string-name>Vito Walter Anelli</string-name>
          <xref ref-type="aff" rid="aff0">0</xref>
        </contrib>
        <contrib contrib-type="author">
          <string-name>Tommaso Di Noia</string-name>
          <xref ref-type="aff" rid="aff0">0</xref>
        </contrib>
        <aff id="aff0">
          <label>0</label>
          <institution>Politecnico di Bari</institution>
          ,
          <addr-line>Bari</addr-line>
          ,
          <country country="IT">Italy</country>
        </aff>
      </contrib-group>
      <pub-date>
        <year>2023</year>
      </pub-date>
      <volume>2870</volume>
      <fpage>22</fpage>
      <lpage>26</lpage>
      <abstract>
        <p>In the era of daily information overload, personalized retrieval applications represent a crucial solution to provide suggestions to users. The research and industrial community have devoted an unprecedented efort to propose approaches and architectures to extract relevant and tailored information from every shred of knowledge. Inspired by the advances in knowledge graph, Graph Convolutional Networks, Link Prediction, and Recommender Systems research, this study aims to meet their cutting-edge research needs of holistic datasets by largely expanding the information available for two well-known recommendation datasets in the book and movie domains, i.e., LibraryThing and MovieLens 25M. We collect the associated knowledge graphs (KGs) for each of them and publish a mapping for each item in the two datasets with the corresponding entities in the original Wikidata, DBpedia, and Freebase KGs. Our work is available at https://github.com/sisinflab/Augmented-and-Linked-Open-Datasets-for-Recommendation.</p>
      </abstract>
      <kwd-group>
        <kwd>eol&gt;Recommender Systems</kwd>
        <kwd>Dataset</kwd>
        <kwd>Knowledge Graphs</kwd>
      </kwd-group>
    </article-meta>
  </front>
  <body>
    <sec id="sec-1">
      <title>1. Introduction and motivation</title>
      <p>and state-of-the-art. Knowledge graphs (KGs) have
the advantage of covering a variety of heterogeneous
Recommender Systems (RSs) constitute the backbone domains, thus making it easier to transpose the advances
of online platforms, pursuing the purpose of suggest- in one domain to another. The adoption of knowledge
ing to customers novel products or services that might graphs as a source of structured information has
genermeet their interests and needs. The pivotal role of rec- ated several advancements in the tasks strictly related to
ommender systems [1] has been advantaged by the no- recommendation [4]: knowledge completion [5],
prefertable accuracy provided by collaborative filtering meth- ence elicitation [6], and user modeling [7, 8]. Given the
ods [2]. Collaborative filtering (CF) is the most tradi- graph nature of data, several diferent recommendation
tional and still prominent recommendation approach. CF approaches have been proposed to tackle: knowledge
models calculate recommendations leveraging similari- transfer across domains [9],
interpretable/explainableties in interaction/preference patterns of similar users. recommendation [10, 11, 12], user modeling [7, 13, 14],
However, recommender systems generally have to face and graph-based recommendation [15, 16, 17, 18, 19].
Aldata sparsity issues due to the availability of little infor- though the KGs are all diferent, every method needs two
mation about users’ preferences, which often causes the side information: a mapping that establishes a
connecso-called cold-start problem. To compensate for this lack tion between items within the catalog and entities in the
of information, an increasing number of approaches com- KGs, and a set of RDF triples that utilize this mapping to
bine collaborative information with auxiliary content describe the relevant domain.
attributes, such as tags, metadata, and geographical data. Unfortunately, dealing with these information sources
Content-based Filtering (CBF) models assemble a pro- raises two important issues for researchers. First, it is
ifle of user interests based on the description of the items usually a time and computationally expensive task due to
that the user has previously consumed. Even though one the limits and constraints of KG services that make this
could leverage this information using traditional CBF operation frustrating. For instance, the public DBpedia
approaches like Vector Space Models and multimedia sig- Virtuoso SPARQL endpoint sets limits both on the rate
nals [3], knowledge graphs gave birth to specific research of requests (whose violation could lead to a permanent
directions that comprise their own literature, models, ban) and on the maximum SPARQL query solution size1.
Second, the information available on the Web rapidly and
constantly changes over time. This causes the diferent
retrieval operations of item attributes to be inconsistent
and lead to diferent outcomes in recommendation
research, thus dangerously jeopardizing the reproducibility
of experiments [20, 21].</p>
      <p>This study aims to tackle these issues by releasing an
1https://www.dbpedia.org/blog/new-dbpedia-usage-report-2021/
extensive collection of side information exploited in tasks and exploit implicit entity preferences to infer the
plauinvolving KGs. We focus on two well-known recommen- sibility of KG facts. Wang et al. [33] explore the intents
dation datasets in the community and detail the steps to behind a user-item interaction by using auxiliary
informagather the collection. In detail, the contribution of the tion in Amazon Books, LastFM and Alibaba-iFashion [34].
work at hand is manifold: Sun et al. [35] present an approach for KG embedding
able to model and infer various relation patterns. They
• a mapping between the items in the LibraryThing evaluate the proposed model on four widely used KGs,
(LT) and MovieLens 25M (ML25M) catalogs and the i.e., FB15k [36], WN18 [36], FB15k-237, and WN18RR.
entities available in three well-known knowl- Zhang et al. [37] use MovieLens 1M and IntentBooks [38]
edge graphs: Wikidata [22], DBpedia [23], and Free- to evaluate CKE, a knowledge-aware model extracting
base [24]; feature representations from a knowledge base and
cap• the retrieval of DBpedia and Wikidata KG’s tures relationships between users and items. This large
triples up to two hops to collect structured informa- amount of work using side information and enriched
tion connected to these resources, thus providing per- datasets shows the potential of these tools. However,
sistent and ready-to-use enriched datasets for perform- often these works use datasets whose linking strategies
ing reproducible experiments; are highly diferent from each other and lack details for
their reproducibility, putting reuse within other works
• a methodology for datasets’ items linking as an at risk.</p>
      <p>approach to promote the emergence of other state-of- For instance, concerning MovieLens and LibraryThing,
the-art enriched datasets; which we treat in this paper, few significant attempts
exist to link these datasets to KGs. Wang et al. [39] link
• an evaluation of the linking methodology by com- MovieLens 1M with IMDb information, such as genres,
paring it with a state-of-the-art entity linker showing actors, directors, and writers to use it in a
knowledgethat traditional linking approaches are inefective for aware model. Ostuni et al. [40] publish a mapping of
this task. MovieLens 1M to the DBpedia KG, while Zhao et al. [41]
The remainder of this paper is organized as follows. propose KB4Rec, a public linking of MovieLens 20M,
Section 2 surveys the existing dataset and the knowledge- LastFM 1B, and Amazon Book to the popular KB Freebase.
aware recommendation works that use them. In Sec- Moreover, to the best of our knowledge, only Di Noia
tions 3 and 4, we describe the outcome of our work, et al. [42] in 2016 succeded linking LibraryThing to a
along with details about challenges we have faced and KG, specifically to DBpedia, likely due to some domain
methodologies we have adopted. In Section 5, we eval- limitations detailed in Section 4.
uate the adopted text similarity-based solution for our These eforts are often not complemented by details
work. Finally, Section 6 concludes this paper, proposing about the adopted linking methodologies and do not
proand discussing future directions. vide an evaluation of their outcome. In addition, previous
literature provides linked datasets but does not publicly
release the RDF triples retrievable by exploring the KGs.
2. Knowledge Graphs in This requires the researchers to scan the KGs each time,
Recommendation Works thus causing high bandwidth, resource, and time
consumption. It is worth highlighting that entities and triples
According to Sun et al. [25], the ten most used datasets in of the KGs change over time, thus producing a mismatch of
the recent literature (sorted in descending order of adop- the retrieved triples that causes reproducibility issues.
tion) are ML-1M [26], Netflix, LastFM, Yelp, ML-20M, These drawbacks have animated us to release extensive
Epinions [27], ML-10M, ML-100K, Book-Crossing [28], linkings of the large MovieLens 25M and LibraryThing
Amazon Music. Unsurprisingly, they relate with stream- datasets to DBpedia, Wikidata, and Freebase and describe
ing and e-commerce platforms and cover the typical do- our methodology in detail. We provide researchers with
mains of movies, music, books, and shops. When linked a vast, up-to-date, and consistent-over-time collection
data is available, it can be leveraged and adopted for ad- of RDF triples. The proposed methodology permits the
dressing several tasks. For instance, Piao and Breslin [29] reproducibility of the results and helps researchers in
link MovieLens 1M and DBook datasets to the DBpedia extracting novel datasets for other domains. Finally, we
KG and investigate the idea of transfer learning between propose an evaluation of our linking strategy, thus
enitem recommendations and KG completion tasks leverag- hancing the trust of the research community in using the
ing a co-factorization model. Also Vagliano et al. [30] and brand-new enriched datasets.</p>
      <p>Alshammari and Nasraoui [31] use knowledge-aware
versions of MovieLens for powering their models. He et al.
[5] adopt in their model the public linking KB4Rec [32]</p>
    </sec>
    <sec id="sec-2">
      <title>3. Dataset Enrichment</title>
      <p>One of the main contributions of this work is providing
the community with two enriched versions of MovieLens
25M (ML25M) and LibraryThing (LT), thus guaranteeing
the reproducibility of any experiment making use of
additional knowledge. Table 1 reports the the overall statistics
for the original datasets, while Table 2 summarizes the
collected resources for both datasets. Each resource is
extensively detailed in the GitHub repository.</p>
      <sec id="sec-2-1">
        <title>MovieLens 25M.</title>
        <p>The MovieLens datasets are the
result of a collective efort of thousands of people
expressing explicit preferences about movies [26]. These datasets
constantly increase in size regarding ratings, users, and
items, with the release that we hereby consider including
about 25 million ratings. We have complemented the
data, DBpedia, and Freebase knowledge graphs. With
this information available, we have explored the
Wikidata and DBpedia graphs up to the second hop [43, 44]
by querying the public SPARQL endpoints to retrieve the
subgraphs originating from the entities associated with
the dataset items.
ing the item ID and the URIs referring to the same entity
in the three knowledge bases. On the other hand, the
facts retrieved from the knowledge graphs have been
( −</p>
        <p>→−
work (RDF). RDF provides a simple data model to encode
knowledge in a structured way through a set of triples in
the form ⟨, ,</p>
        <p>⟩. Each triple represents the connection
 between the subject  and the object  through
a predicate  . We define</p>
        <p>exploration at 1-hop of  as
the operation of retrieving the triples having  as the
subject. Generalizing, the exploration at -hop is the
operation to retrieve the triples having as subjects all
the objects obtained at the exploration from 1-hop to
1)-hops. In the proposed resources, we collected
the triples obtained exploring DBpedia and Wikidata at</p>
        <sec id="sec-2-1-1">
          <title>1-hop and at 2-hops.</title>
          <p>LibraryThing. The LT dataset has been collected in
2011 from the namesake website, which allows users to
items in MovieLens 25M with links to resources of Wiki- In this section, we present methodologies, challenges,
create a virtual catalog of the books they own or have
read. The dataset contains item reviews with ratings from</p>
        </sec>
        <sec id="sec-2-1-2">
          <title>1 to 5 along with the number of "helpfulness" votes they</title>
          <p>received. The linking of LibraryThing items to DBpedia
and Wikidata URIs followed a similarity-based approach,
due to some information mismatching that we discuss in
Section 4 and that made dificult to obtain a definite
mapping. As a consequence, the item-URI linking comes with
a similarity, that accounts for a score that a book
actually corresponds to the associated resource on Wikidata
and DBpedia. Hence, one can set a confidence
threshold, bearing in mind that the lower is the threshold, the
higher is the number of linked entities, but the higher is
the occurrence of false positives. Although a threshold in
the range of [0.5, 0.6] may represent an acceptable
tradeof between true positives and false positive,
we have
not fil tered the retrieved information, leaving each
researcher the choice to select it. Regardless of the linking
similarity scores, we have linked the items to Freebase
entities. Additionally, we have provided all the triples
retrievable at July 2023 from DBpedia and Wikidata by
querying the SPARQL endpoints using the linked entities
and exploring the graphs at the first and at the second
hop.</p>
        </sec>
      </sec>
    </sec>
    <sec id="sec-3">
      <title>4. Item Linking Methodology</title>
      <p>and solutions adopted for performing the item linkings
described in Section 3. Our goal is to sketch best practices
and highlight issues that raise from item linking, with
the aim of encouraging the discussion about the need of
a common workflow to collect augumented datasets.</p>
      <sec id="sec-3-1">
        <title>The goal of an item linker is to connect the items of a</title>
      </sec>
      <sec id="sec-3-2">
        <title>Open Data (LOD) Knowledge Base. With this step, we</title>
        <p>try to fill the gap between the items in the catalog and</p>
      </sec>
      <sec id="sec-3-3">
        <title>LOD cloud. In this respect, we could adopt two diferent</title>
        <p>item description linking, as suggested by Di Noia [45].</p>
        <sec id="sec-3-3-1">
          <title>MovieLens 25M.</title>
        </sec>
      </sec>
      <sec id="sec-3-4">
        <title>MovieLens 25M is provided with</title>
        <p>a map from internal item IDs to the identifiers of the
movies in the massive Internet Movie Database (IMDb)
and in The Movie Database (TMDb). We have exploited
this connection and adopted a direct item linking strategy,
querying the Wikidata endpoint with a SPARQL query.</p>
      </sec>
      <sec id="sec-3-5">
        <title>Specifically, we retrieve each item resource URI in Movie</title>
      </sec>
      <sec id="sec-3-6">
        <title>Lens 25M by performing at least one SPARQL query: (i)</title>
        <p>the first query uses the IMDb identifier to find the
corresponding entity on Wikidata; (ii) if the first attempt
does not succeed, another query is sent to the endpoint
using the TMDb identifier. Once the resource has been
found in Wikidata, we reconstruct the corresponding</p>
        <p>On the one hand, item linkings are collected compris- catalog with the corresponding resources in the Linked
provided in the standard Resource Description Frame- solutions to address this goal, i.e., direct item linking and
DBpedia URI from the English Wikipedia page URL. The instance  ∈  with its title  ∈  and its author
linking approach chosen for MovieLens 25M does not  ∈ , we aim to create a direct mapping  → ,
generate false positives thanks to the accurate references where  ∈  is characterized by a title  ∈ 
from MovieLens 25M items to IMDBb and TMDb IDs, and an author  ∈ , based upon the similarities
and to the explicit reference to those IDs in the Wiki- between titles labels  and  and authors labels  and
data Knowledge Graph. However, for each item in the .
dataset, at least one SPARQL query had to be performed. To this end, we consider and combine three popular
Therefore, the researcher should take into account the costs similarity metrics, namely the Jaro [46], Ratio [47], and
for establishing a huge number of HTTP requests and the -grams [48] similarities. We assume string similarity
limitations that an endpoint may place. could be a reasonable solution to ensure that two
entities are the same. Furthermore, by calculating and
LibraryThing. LibraryThing collects books using in- combining the similarities between title names and
auternal identifiers without sharing the international iden- thors, we may address critical situations where titles
tifier or a website page. Moreover, books are usually sold are the same, but authors do not match, and vice versa.
in diferent editions and LibraryThing manages this in- Then, by considering two strings 1 and 2, we define
formation treating them as completely diferent books. _(1, 2) as the average of the these similarity
Although book entities in Wikidata and DBpedia are gen- measures. Concerning the  and  sets, and
takerally linked to a page of the LibraryThing website, a ing into account an instance  ∈  and an instance
relevant issue comes from the fact that the LibraryThing  ∈  , we perform _(, ) (i.e., the
similardataset and the Wikipedia/DBpedia KGs often link to ity between title labels) and _(, ) (i.e., the
diferent editions of the same book — thus generating a similarity between author labels). Subsequently, we
mulmismatching of the identifiers. To overcome this issue, tiply them to obtain the combined similarity measure
we adopt the item description linking strategy. With this (, ) = _(, ) * _(, ). We set
approach, we exploit a similarity function to compare a _(, ) = 1 if the labels of the author  and/or
resource’s title and author available in the dataset with  are missing, so that the title labels are the only ones
the corresponding information of the books from Wiki- to contribute to the similarity since 1 is the neutral factor
data or DBpedia. Since the similarity is computed with for the multiplication. We perform this operation for
a dedicated ofline operation, this approach requires to each instance  ∈  , obtaining a number of triples
query only once Wikidata and DBpedia to retrieve all the  ≜ (, , (, )) equals to | | × | |. Finally, we
books in these KGs along with their titles and authors. select at most one triple  with max((, )) for each
The linking task is then realized with a post-hoc anal- combination of  and . In this respect, we obtain a direct
ysis. To summarize, we created the collection  of the and unambiguous mapping between the book ID of  and
books’ URI instances available on Wikidata and DBpedia the URI in . However, the linking may not be unique at
with their titles () and authors (). Then, we created this stage because some diferent URIs may be associated
the collection  of books IDs from LibraryThing with with the same ID. In such cases, we make the linking
their titles () and authors (). Finally, given an unique by only considering the one characterized by the</p>
        <p>From DBpedia and Wikidata to Freebase. Both the
DBpedia and Wikidata KG provide between their entities
and Freebase KG. Therefore, we get from the both KGs
all the object entities referenced by these predicates to
accomplish this linking stage. As for MovieLens 25M, a
Freebase reference was searched by exploring DBpedia,
while for the entities not linked to Freebase yet, a
reference was searched by querying Wikidata. Concerning
LibraryThing, the solution adopted to perform the item
linking led us to generate two diferent comprehensive
resources, one for each KG.
uation). Finally, we link a URI resource of DBpedia to the
iflm having the highest similarity score and compare this
linking with the ground truth. It is worth mentioning that
side information could be diverse even though it refers to
the same subject. For clarity, one can easily understand
that "John Lasseter" and "J. Lasseter" are correct
information about Toy Story’s director. However, even though
5. Evaluation this situation is common, an automated agent finds it
more challenging to identify the same entity. Hence, we
This section describes the limitations of existing named repeated the experiment to evaluate the robustness of
entity linkers [49, 50, 51] for this linking task and the the linkers by introducing a random noise on the strings.
need for an ad-hoc linker for the book domain. Then, an Specifically, each character is deleted e/o perturbed with
experimental evaluation is conducted to assess if the pro- a probability of 0.15.
posed solution can overcome the performance of existing The results in Table 3 show the eficacy of the
prolinkers. In detail, diferently from a traditional linking posed solution. With the support of the original side
task where a textual context is provided, only titles and information only, our linker successfully links 40 189 of
authors are retrieved from LibraryThing. Although this the items, an acceptable performance compared to
DBpecould already be a reasonable motivation, we compare dia Spotlight (15 483). For what concerns the robustness
the solution with a state-of-the-art linker, DBpedia Spot- analysis, DBpedia Spotlight performance collapses up
light, a framework for automatic annotation of DBpedia to 1755 of correct links, while the proposed linker loses
entities from natural language text [50, 52]. only 4550 links. In conclusion, the results justify the</p>
        <p>However, a ground truth is needed to evaluate the adoption of a tailored linker for this peculiar application
methods. Unfortunately, LibraryThing does not provide scenario.
this information, but MovieLens does. In fact, the
MovieLens dataset contains titles, IMDb, and TMDb IDs.
Therefore, we can exploit it to evaluate the performance of the 6. Conclusion and Future Work
linkers by fixing the available side information, i.e., the
label of the film and its director taken from IMDb. This work provides supplementary data of two
well</p>
        <p>On the one hand, to get DBpedia Spotlight working known and widely used recommendation datasets (i.e.,
at its best, we supply this framework with a context by MovieLens 25M and LibraryThing) that can be utilized
generating sentences exploiting the available side infor- in tasks involving Knowledge Graphs. Inspired by the
mation. For instance, given as metadata the film title " Toy new information needs of cutting-edge research, we
proStory" and its director "John Lasseter", we ask DBpedia vide mappings to three well-known Knowledge Graphs:
spotlight to annotate DBpedia entities in the sentence DBpedia, Wikidata, and Freebase. To ensure
reproducibil"The film Toy Story is directed by John Lasseter ". Then, DB- ity of the future experiments, we also provide domain
pedia Spotlight should be capable of taking out, among Knowledge Graphs at the first and second hop of
explothe other recognized entities, the resource URI in the ration in the form of RDF triples, thus enabling reliable
DBpedia Knowledge Graph (KG) of the film " Toy Story", Graph-based and Knowledge-aware recommendation
rewhich should correspond to the one found with direct search. Furthermore, they are aligned and expose the
item-linking. On the other hand, we follow the pipeline same additional information to ease the researcher’s
efproposed for the book domain (see Section 4), retrieving fort in conducting evaluation and comparisons. For
fuall the films in the DBpedia KG with their labels and direc- ture work, we encourage the enrichment of datasets from
tors (corresponding to the book titles and their authors other domains such as music and shops and their
intein the book domain) and computing string similarities gration with spectrograms and images, respectively, to
with IMDb-derived data (the candidate items in this eval- design novel multi-modal recommendation methods.
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