![Table 2: Topic milestone papers (top-10 papers) for Sentiment Analysis from [8].](/figures/table-2-topic-milestone-papers-top-10-papers-for-sentiment-riuw03tb.webp)
A Collective Topic model for Milestone Paper Discovery
Ziyu Lu
The University of Hong Kong
Pokfulam Road, Hong Kong
zylu@cs.hku.hk
Nikos Mamoulis
The University of Hong Kong
Pokfulam Road, Hong Kong
nikos@cs.hku.hk
David W. Cheung
The University of Hong Kong
Pokfulam Road, Hong Kong
dcheung@cs.hku.hk
ABSTRACT
Prior arts stay at the foundation for future work in aca-
demic research. However the increasingly large amount of
publications make it difficult for researchers to effectively
discover the most important previous works to the topic of
their research. In this paper, we study the automatic dis-
covery of the core papers for a research area. We propose
a collective topic model on three types of objects: papers,
authors and published venues. We model any of these ob-
jects as bags of citations. Based on Probabilistic latent se-
mantic analysis (PLSA), authorship, published venues and
citation relations are used for quantifying paper importance.
Our method discusses milestone paper discovery in different
cases of input objects. Experiments on the ACL Anthology
Network (ANN) indicate that our model is superior in mile-
stone paper discovery when compared to a previous model
which considers only papers.
Categories and Subject Descriptors
H.3.3 [Information Search and Retrieval]: Information
Search and Retrieval—clustering, retrieval models
Keywords
Topic Model; Milestone Paper; Paper Importance
1. INTRODUCTION
Academic literature surveying plays a vital role in aca-
demic research; researchers can learn what has been done,
what research gaps might exist and what potential research
directions to work on. Academic search engines such as
Google Scholar
1
and CiteSeerX
2
enable researchers to find
related literatures or prior arts. However, the overwhelm-
ing number of publications makes it difficult to quickly ob-
tain the most important set of previous work of a subject.
1
http://scholar.google.com
2
http://citeseerx.ist.psu.edu/index
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Some citation recommendation systems have been designed
to recommend appropriate citations for academic works [5,
1]. However, academic search engines and recommendation
systems might return qualified sets of papers based on se-
mantic similarity; paper importance has not been consid-
ered. It is essential to have some models for milestone paper
discovery; i.e., discover the core set of papers which can best
represent previous works for a research topic. Wang et al.
[8] studied topic milestone paper discovery and developed
a generative model for theme and topic evolution. They
used the idea of modeling each paper as a “bag of citations”
and measured paper impacts based on co-citation relations.
However, [8] only considered co-citation relations for topic
milestone paper discovery. Therefore, their model could be
biased against recently published papers (which are rarely
cited by others). Thus, the issue of milestone paper discov-
ery is not completely addressed.
In this paper, we propose a collective topic model for ob-
jects of different types (i.e., papers, authors, and venues)
in academic networks. The collective topic model quanti-
fies paper importance based on authorship, published venue
reputation and co-citation relationships in the academic col-
lection. We investigate the identification of milestone papers
in different cases. Our experimental results show that paper
importance is well captured by our model; authorship and
published venues have considerable influence on milestone
paper discovery.
2. RELATED WORK
Previous work exists in citation recommendation [5, 1];
i.e., recommending appropriate references to researchers. For
example, [5] designed a translation model between citation
contexts and reference words, and recommended a list of ci-
tations by using long queries such as sentences or a manuscript.
Bethard and Jurafsky [1] designed a feature-based learning
model for literature retrieval. A list of references are rec-
ommended using the abstract of the input object (i.e., a pa-
per) as query. These works perform recommendations based
on semantic analysis, but paper importance or ranking is
not considered. In addition, some works in topic evolution
may enable researchers to see how the research in a par-
ticular area evolves. Mei and Zhai [6] used temporal text
mining techniques to discover latent themes from text and
constructed theme evolution graphs. However, they cannot
identify the “micro-view” of a research field, e.g., milestone
papers for that area.
Wang et al. [8] used milestone paper discovery as an ap-
plication when developing a generative topic model for re-
Figure 1: Overview of our collective topic model
search theme evolution. They modeled a paper as a bag of
citations and use the co-citation relationships for evaluating
paper impact between topics. However, their results can be
biased against recently published papers, since they did not
take into account additional factors that influence the impor-
tance of papers, such as authorship and published venues.
In our work, we propose a topic model that considers these
additional influence factors.
3. PROBABILISTIC TOPIC MODEL
The importance of a paper depends on a variety of factors,
including the authority of authors, the publication venue and
co-citation relationship with other papers. We borrow the
idea of modeling a paper as a bag of citations from [8] in
order to consider the co-citation factor in the importance of
a paper; thus each paper is represented as a bag of citation
IDs. Since authors and venues are linked with documents in
the academic document collection, we build a “virtual doc-
ument” for each author and venue by aggregating all doc-
uments associated with that author or venue (we call the
result author document and venue document, respectively).
This way, for each author or venue we also derive a bag of
citation IDs. Based on [3], we assume that the multiple-
typed documents (paper document, author document and
venue document) have a common set of latent topics and
each topic is represented as the distribution over citations.
We model these documents using a probabilistic topic model
which quantifies the probability of a paper to be cited as pa-
per importance. Then, the problem about milestone paper
discovery is defined as follows.
Milestone paper discovery: Given an academic docu-
ment collection D (papers Q, author set A , published venues
V and citations C are known), the model outputs a set of
core papers M that includes citationIDs (paperID) ranking
based on co-citations at top within a topic or ”a document”
(The document type might be a paper document, an author
document or a venue document).
3.1 Model Description
An overview of our model is shown in Figure 1. Table 1
describes meanings of the notations used in our model. We
assume that for all documents there is a common set of k
latent topics. Each document is represented as the distri-
bution over topics and each topic is represented as the dis-
tribution over citations. Then, the process of generating an
academic document is as follows: for each citation in that
document, firstly sample a topic z
k
according to the distri-
bution from paper topic distribution δ(z; d) or author topic
distribution ζ(z; a) or venue topic distribution ψ(z; v) based
Table 1: Notations used in our collective model
Symbols Description
d, a d for a paper, a for an author
v, c, z v for a venue, c for a citation, z for a topic
T , k T for topic set, k for topic number
N The citation-paper Matrix
U The citation-author Matrix
E The citation-venue Matrix
φ(c; z) The topic-citation distribution
δ(z; d) The paper-topic distribution
ζ(z; a) The author-topic distribution
ψ(z; v) The venue-topic distribution
α, β, γ relative weights for d, a, v
on the document type. Then, draw a citation c from the
sampled topic distribution φ(:; z
k
) in topic citation distri-
bution φ(c; z).
We developed our model based on PLSA [4]. We can have
the following joint model for citations based on documents
in different types:
p(c
i
|d
j
) =
X
k
φ(c
i
; z
k
)δ(z
k
; d
j
) (1)
p(c
i
|a
n
) =
X
k
φ(c
i
; z
k
)ζ(z
k
; a
n
) (2)
p(c
i
|v
m
) =
X
k
φ(c
i
; z
k
)ψ(z
k
; v
m
) (3)
and the parameter set θ to be estimated is:
θ ={φ(c|z), δ(z; d), ζ(z; a), ψ(z; v)
| c ∈ C, d ∈ D, a ∈ A, v ∈ V, z ∈ T }
In order to estimate the parameters θ, we should maximize
the likelihood of the document collection D given θ. The
loglikelihood function is represented as
L(θ) =
X
i
(α
X
j
N
ij
log p(c
i
|d
j
) + β
X
n
U
in
log p(c
i
|a
n
)
+ γ
X
m
E
im
log p(c
i
|a
m
))
N
ij
indicates the occurrences of citation c
i
in paper d
j
in
the citation-paper matrix, U
in
the occurrences of citation c
i
cited by a
n
and E
in
the occurrences of citation c
i
cited by
papers in venue v
m
. α, β, γ indicate relative weights for
three-typed documents d, a, v.
3.2 Parameter Inference
We use the Expectation-Maximization (EM) algorithm for
parameter inference. EM iteratively executes two steps, an
E-step and a M-step, until L(θ) converges [2].
Each E-step computes the lower bound function Q of L(θ).
In this process, the posterior probabilities p(z
k
|c, o) (o can
be d, a, v) are re-computed using the new parameter values
from the previous M-step:
p(z
k
|c
i
, d
j
) =
φ(c
i
|z
k
)δ(z
k
|d
j
)
P
k
φ(c
i
; z
k
)δ(z
k
; d
j
)
p(z
k
|c
i
, a
n
) =
φ(c
i
; z
k
)ζ(z
k
; a
n
)
P
k
φ(c
i
; z
k
)ζ(z
k
; a
n
)
Table 2: Topic milestone papers (top-10 papers) for Sentiment Analysis from [8].
φ(c
i
; z
k
) Venue Paper Title
0.0785 EMNLP’02 Thumbs Up? Sentiment Classification Using Machine Learning Techniques
0.0672 ACL’02 Thumbs Up Or Thumbs Down? Semantic Orientation Applied To Unsupervised Classification Of Reviews
0.0483 HLT’05 Recognizing Contextual Polarity In Phrase-Level Sentiment Analysis
0.0436 ACL’04 A Sentimental Education: Sentiment Analysis Using Subjectivity Summarization Based On Minimum Cuts
0.0365 ACL’97 Predicting The Semantic Orientation Of Adjectives
0.0312 COLING’04 Determining The Sentiment Of Opinions
0.0307 HLT’05 Extracting Product Features And Opinion From Reviews
0.0287 EMNLP’03 Towards Answering Opinion Questions: Separating Facts From Opinions And Identifying The Polarity Of Opinion Sentences
0.0279 EMNLP’03 Learning Extraction Patterns For Subjective Expressions
0.0169 ACL’05 Seeing Stars: Exploiting Class Relationships For Sentiment Categorization With Respect To Rating Scales
Table 3: Topic milestone papers (top-10 papers) for Sentiment Analysis in our collective model.
φ(c
i
; z
k
) Venue Paper Title
0.1317 EMNLP’02 Thumbs Up? Sentiment Classification Using Machine Learning Techniques
0.0747 ACL’04 A Sentimental Education: Sentiment Analysis Using Subjectivity Summarization Based On Minimum Cuts
0.0689 ACL’02 Thumbs Up Or Thumbs Down? Semantic Orientation Applied To Unsupervised Classification Of Reviews
0.0482 HLT’05 Recognizing Contextual Polarity In Phrase-Level Sentiment Analysis
0.0351 ACL’05 Seeing Stars: Exploiting Class Relationships For Sentiment Categorization With Respect To Rating Scales
0.0304 ACL’07 Biographies, Bollywood, Boom-boxes and Blenders: Domain Adaptation for Sentiment Classification
0.0215 ACL’07 Structured Models for Fine-to-Coarse Sentiment Analysis
0.0210 EMNLP’08 Learning with Compositional Semantics as Structural Inference for Subsentential Sentiment Analysis
0.0195 EMNLP’08 Multilingual Subjectivity Analysis Using Machine Translation
0.0177 ACL-IJCNLP’09 Co-Training for Cross-Lingual Sentiment Classification
170000
180000
190000
200000
210000
220000
230000
240000
250000
260000
270000
280000
290000
300000
1 21 41 61 81 101 121 141 161 181
Perplexity
Iteration Number
k=50
k=100
k=120
k=150
k=200
k=300
k=500
Figure 2: The perplexity over different k for our
model
p(z
k
|c
i
, v
m
) =
φ(c
i
; z
k
)ψ(z
k
; v
m
)
P
k
φ(c
i
; z
k
)ψ(z
k
; v
m
)
In the first E-step, the posterior probabilities are ran-
domly initialized. The M-step that follows each E-step, re-
estimates the θ which maximizes Q as follows:
δ(z
k
; d
j
) =
P
i
N
ij
p(z
k
|c
i
, d
j
)
P
i
N
ij
ζ(z
k
; a
n
) =
P
i
U
in
p(z
k
|c
i
, a
n
)
P
i
N
in
ψ(z
k
; v
m
) =
P
i
U
im
p(z
k
|c
i
, v
m
)
P
i
N
im
φ(c
i
; z
k
) ∝α
P
j
N
ij
p(z
k
|c
i
, d
j
)
P
i
P
j
N
ij
p(z
k
|c
i
, d
j
)
+ β
P
n
U
in
p(z
k
|c
i
, a
n
)
P
i
P
n
U
in
p(z
k
|c
i
, a
n
)
+ γ
P
m
E
im
p(z
k
|c
i
, v
m
)
P
i
P
m
E
im
p(z
k
|c
i
, v
m
)
4. EXPERIMENTS
4.1 Dataset
The ACL Anthology Network (ANN) [7] was used in our
experiments. There are 19408 papers by 15824 authors, pub-
lished in 342 venues and having received 85367 citations.
This dataset is also used in previous work [8]; thus, we can
use it to perform some comparisons with [8].
Before testing our model, we have to determine the num-
ber of topics k. Perplexity is commonly used to evaluate
performance of topic modeling. Thus, we select the value
of k which minimizes perplexity. Figure 2 shows the per-
plexity scores during model estimation for different values
of k. From this graph, we can see that a value of k around
150 is appropriate for this dataset, since it gives the low-
est perplexity score among all tested values. Therefore, we
perform our experiments using k = 150. In addition, we con-
sider the three-typed documents equally and set the weights
α = β = γ = 1.
4.2 Experimental Results
4.2.1 Results of Topic Milestone Paper Discovery
Each topic is presented as the mixture of citations in our
model. Within each topic z
k
, φ(c
i
; z
k
) indicates the impor-
tance of citation (i.e., the cited paper) c
i
. Those citations
can be ranked based on φ(c
i
; z
k
) and citations ranking at
the top for each topic z
k
are considered as topic milestone
papers. In order to compare with previous work [8], we use
the top-10 papers for the topic Sentiment Analysis as an
example. Table 2 presents topic milestone papers for Senti-
ment Analysis in [8] while Table 3 shows our results. There
are 5 overlapping papers and 5 different papers between the
two models. The top-1 paper is the same, but the order of
the overlapping papers is slightly different. The top-2 pa-
per in our model is ranked highly, compared with that in [8]
(top-4). We have more recently published papers. The rea-
son behind these differences is that the previous model does
not consider factors such as published venues and authorship
that have influence on paper importance.
4.2.2 Results of Venue Milestone Paper Discovery
In previous work, only topic milestone paper discovery has
been studied. Our model is more general in the sense that
it can identify milestone papers also for a given venue or
author. In the next experiment, the probability of a cita-
tion given a venue is computed by Equation 3 and papers
are ranked based on p(c|v). Here we only take the venue
Table 4: Milestone papers (top-10) for ACL in our collective model.
p(c|v) Paper Title Venue
0.0084 Building A Large Annotated Corpus Of English: The Penn Treebank JCL’93
0.0074 The Mathematics Of Statistical Machine Translation: Parameter Estimation JCL’93
0.0059 Statistical Phrase-Based Translation NAACL’03
0.0057 Bleu: A Method For Automatic Evaluation Of Machine Translation ACL’02
0.0056 A Hierarchical Phrase-Based Model For Statistical Machine Translation ACL’05
0.0056 A Systematic Comparison Of Various Statistical Alignment Models JCL’03
0.0050 Minimum Error Rate Training In Statistical Machine Translation ACL’03
0.0050 A Maximum-Entropy-Inspired Parser NAACL’00
0.0044 Stochastic Inversion Transduction Grammars And Bilingual Parsing Of Parallel Corpora JCL’97
0.0044 Accurate Unlexicalized Parsing ACL’03
Table 5: Author milestone papers (top-10) for the author Bo Pang.
p(c|a) Paper Title Venue
0.0461* Thumbs Up? Sentiment Classification Using Machine Learning Techniques EMNLP’02
0.0375 Thumbs Up Or Thumbs Down? Semantic Orientation Applied To Unsupervised Classification Of Reviews ACL’02
0.0253* A Sentimental Education: Sentiment Analysis Using Subjectivity Summarization Based On Minimum Cuts ACL’04
0.0241 Predicting The Semantic Orientation Of Adjectives ACL’97
0.0172 Extracting Product Features And Opinions From Reviews HLT’05
0.0152 Towards Answering Opinion Questions: Separating Facts From Opinions And Identifying The Polarity Of Opinion Sentences EMNLP’03
0.0147 Learning Extraction Patterns For Subjective Expressions EMNLP’03
0.0093* Seeing Stars: Exploiting Class Relationships For Sentiment Categorization With Respect To Rating Scales ACL’05
0.0086 Biographies, Bollywood, Boom-boxes and Blenders: Domain Adaptation for Sentiment Classification ACL’07
0.0065 Learning Subjective Language JCL’04
ACL as example and Table 4 reports important papers in
ACL. These papers are mainly from three venues, e.g ACL,
NAACL and JCL (Journal of Computational Linguistics).
4.2.3 Results of Author Milestone Paper Discovery
Similarly, author milestone papers can be ranked, based
on the probability of a citation given an author p(c|a) (see
Equation 2). We used the author Bo Pang (the first author
of the top-1 paper in Table 3) as an example. Table 5 shows
the citations that Bo Pang has the highest probability to
cite (* indicates self-citation). The result has high overlap
with Table 2 (8 papers). This might indicate that author ci-
tation patterns are regular and the author factor has similar
influence as the citation relationships.
4.2.4 Topic Involvements
Finally, our model can also indicate popular topics for
an author or a venue. ψ(z; v) indicates how well a topic z
represents a venue v and ζ(z; a) shows how well a topic z
can represent an author a. Therefore, we can find the most
popular topics for a specific venue or an author. The top-
3 topics for ACL are respectively Name entity extraction,
Statistical parsing and Statistical machine translation. The
top-3 topics for Bo Pang are Sentiment analysis, Opinion
extraction and Paraphrases generation.
5. CONCLUSIONS AND FUTURE WORK
In this paper, we proposed a collective topic model for
multiple-typed objects in the academic network, in order to
address the issue of milestone paper discovery. The model is
based on PLSA, and authorship, published venues and cita-
tion relations have been included in it. Our method can not
only discover topic milestone papers discussed in previous
work, but also explore venue milestone papers and author
milestone papers. In addition, it can find representative top-
ics for an author or a venue. Experiments on a real dataset
ANN show that our model can better evaluate the impact
of papers and its result is not biased against new publica-
tions. Directions for future work include the investigation of
more complicated models with biased mechanisms and the
integration of this model into existing academic literature
search/recommendation systems.
6. ACKNOWLEDGMENTS
This work was supported by grant HKU 715711E from
Hong Kong RGC. The authors would like to thank the anony-
mous reviewers for their valuable comments.
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