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Diversifying Search Result Leveraging Aspect-based Query Expansion.


Web search has become the predominant method for users to fulfill their information needs. In this aspect, users describe their information needs by providing a set of keywords. These keywords are collectively called a search query for each user. Since expressing information need through keywords is difficult, some users fail to choose the precise terms while others tend to omit important terms needed to clarify search intentions [1, 2]. Therefore, a large number of the web search queries are usually short, ambiguous, and prone to have multiple interpretation [3, 4, 5]. Generally, the short queries mean a lot of ambiguity as to what information needs the users express. Consider a short and ambiguous query "Java", which could be interpreted as a programming language, island, coffee, etc.

For such type of queries, the search engine may generate a ranking of documents with maximum redundancy covering a very few user information needs. To mitigate these issues, search result diversification (SRD) can be used to generate the effective ranking of documents. Some clustering algorithms applied in different perspectives [6, 7] can be used for SRD. Diversification approaches re-rank the retrieved documents considering intents or aspects for the user query. Therefore, the retrieved documents contain less redundant documents. In turns, the retrieved documents also cover user query aspects as much as possible. The common principle used in the existing SRD approaches is to select as diverse results as possible from a given set of retrieved documents. The final ranking list is much dependent on the initial retrieval results, which may not have a good coverage of the different aspects of the query. To overcome this drawback, some existing studies on SRD attempted to expand the original query before diversifying the results [8, 9].

Query expansion is a classic technique to reformulate the query, which generates diversified expansion terms to enhance the original query. There are lot of approaches to expand the original query using different resources and techniques including pseudo-relevance feedback [10], word-embedding [11, 12, 13], ConceptNet and WordNet [14], and Freebase [15] etc. Query expansion techniques are widely applied for improving the efficiency of the textual information retrieval systems. These techniques help to overcome vocabulary mismatch issues by expanding the original query with additional relevant terms and reweighing the terms in the expanded query.

In this research, we propose an aspect-based query expansion technique to diversify the retrieved documents for the original query. Query suggestions and completions from search engines are good resources to reformulate the original query. Therefore, our proposed approach is to retrieve query suggestions and completions for each query from three commercial web search engines namely, Google, Yahoo, and Bing. The aggregated list of suggestions and completions are used as a resource to expand the original query. A frequent phrase based soft clustering algorithm is then applied to group similar candidates into clusters. Here every cluster represents different query aspect. The generated cluster labels are then used to expand the query. We employ all the terms from the cluster labels except query terms and stop words to expand the query. Finally, the retrieved documents are re-ranked based on the relevancy with the expanded query. To estimate the relevancy between web document and the query, we propose multiple semantic and lexical features using word-embedding and their content information, respectively. We conducted experiments using the Clueweb09 document collection with TREC 2012 Web Track query set. The experimental results clearly illustrate that our proposed aspect-based query expansion method is effective to diversify web documents. There are two distinct contributions in proposed method:

1. A novel query expansion technique based on users' aspect and

2. Multiple new semantic and lexical features to estimate the relevance between expanded query and documents

The rest of the paper is structured as follows: In section 2, we summarize related work on query expansion and document retrieval. In section 3, we briefly explain two classical retrieval model. We present our proposed method in section 4. The experiments and evaluation to show the effectiveness of our proposed method is presented in section 5. Some concluded remarks and future directions are described in section 6.


Usually, queries to web search engines are short and not written carefully, which makes it more difficult to understand the intent behind a query and retrieve relevant documents. A common solution is query expansion, which uses a larger set of related terms to represent the user's intent and improves the documents' ranking.

Pseudo Relevance Feedback (PRF) algorithms are widely used in query expansion. These algorithm assume that top ranked documents for the original query are relevant that contain good expansion terms. The researchers proposed a model that selected expansion terms based on their term frequency in top retrieved documents, and weights them by documents' ranking scores [16]:

[mathematical expression not reproducible]

where D is the set of top retrieved documents, p(t|d) denotes the probability that term t that generated by document d's language model, and f(q; d) denotes the ranking score of the document provided by the retrieval model. Later, another study [17] added inverse document frequency (IDF) to demote very frequent terms:

[mathematical expression not reproducible]

where p(t|C) denotes the probability of term t in the corpus language model C.

Another PRF approach has also been proposed using a Mixture Model [18]. In that study, the researchers assumed that the terms in top retrieved documents are drawn from a mixture of two language models: query model [[theta].sub.q] and a background model [[theta].sub.B]. The likelihood of a top retrieved document d is defined as follows:

[mathematical expression not reproducible]

[[alpha].sub.d] denotes a document-specific mixture parameter. For this equation, the query model [[theta].sub.q] can be learned by maximizing the top retrieved documents' likelihood. The terms that have non-zero probability in [[theta].sub.q] are used for query expansion.

Knowledge base such as Freebase can also be applied by query expansion methods [15]. Those methods identified the entities associated with the query, and used the entities to perform query expansion. A supervised model combined information derived from Freebase descriptions and categories to select terms that are effective for query expansion. The researchers also proposed a method to expand the original query with the help of WordNet and ConceptNet [14]. Their approach extended the query with the synonyms generated from WordNet.

Recently, word-embedding techniques are used for query expansion [11, 12, 13]. An Automatic Query Expansion (AQE) framework has been proposed by using distributed neural language model, word2vec [13]. They trained a word2vec model that learned a low dimensional embedding for each vocabulary entry using the semantic and contextual relation in a distributed and unsupervised approach. They selected the related terms to the query by applying a K-nearest neighbor technique and those terms were used for expansion. A query expansion technique is introduced for adhoc retrieval using a locally trained word embedding model [11]. They presented local embedding which capture the nuances of topic-specific language better than global embeddings. Another study also proposed wordembedding based method for query expansion [12]. They applied continuous-bag-ofwords implementation of word2vec over the entire corpus on which search is performed and selected terms that are semantically related to the query. Their method either used the terms to expand the original query or integrate them with the effective pseudo-feedback-based relevance model.


In this section, we discuss two classical retrieval models, Okapi BM25 [19] and Language model [16]. In this research, these two classical retrieval models are utilized as baseline document retrieval.

3.1 Okapi BM25 Model

Let d be an unstructured document in the collection C. We may consider this as a vector [mathematical expression not reproducible] = (t[f.sub.1]; ... ; t[f.sub.V]), where t[f.sub.i] denotes the term frequency of the i-th term [t.sub.i] in the document d and V is the total number of terms in the vocabulary. In order to score such a document against a query, most ranking functions define a term weighting function [w.sub.j]([mathematical expression not reproducible],C). BM25 is an example of such functions. For ad-hoc retrieval, and ignoring any repetition of terms in the query, BM25 [19] can be simplified as follows:

[mathematical expression not reproducible]

where d[f.sub.j] is the document frequency of j-th term, dl is the document length, avdl is the average document length in the collection, and [k.sub.1] and b are tuning parameters.

The document score is then obtained by adding the document term weights of term matching the query q:

[mathematical expression not reproducible] (2)

3.2 Language Model

Language model is a quite general formal approach in information retrieval. Query likelihood model is the most basic method for using language models in information retrieval. Let us assume a simple unigram for each document, where each document is represented as the standard bag-of-words and their language model is distributed over a vocabulary of a single word. The maximum likelihood estimate of term w occurring in document d for a multinomial distribution is given below [16]:

[mathematical expression not reproducible] (3)

where t[f.sub.w,d] is term frequency (number of times term w appears in document d) of the term w in document d and |d| denotes the total number of terms in d.

Given a query q = {[q.sub.1], [q.sub.2]; [q.sub.3]...[q.sub.k]}, the likelihood can be computed for the document d as follows:

[mathematical expression not reproducible] (4)

This likelihood is computed for each document and used for ranking. Ranking documents in this procedure is known as query likelihood language model.

The smoothed p(w|d) with Jelinek-Mercer smoothing is estimated as follows:

[mathematical expression not reproducible] (5)

where cfw is the term frequency of term w in the collection C and [lambda] is a smoothing parameter.


This section presents our proposed aspectbased query expansion method for search result diversification. For a given query, our method produces a diversified list of documents using aspect-based query expansion. The high-level building blocks of our proposed method are illustrated in Fig. 1. There are two major parts in our method, query expansion and search result diversification using the expanded query.

Given a query, the query expansion technique expands the original query covering user query aspects as much as possible. In this regards, we propose a new aspect-based approach to select expansion terms. The search result diversification technique returns a list of diversified documents with respect to the expanded query. In this part, multiple new semantic and lexical features are introduced to estimate the relevancy between the query and document. The remainder of this section presents the complete explanation of the query expansion and the search result diversification techniques.

4.1 Aspect-based Query Expansion

Generally, search queries are very short in length. The existing study on query structure suggested that, the average length of search queries is around 2.3 terms per query [20]. Usually, the short queries mean a lot of ambiguity as to what information needs the users express. Therefore, we reformulate the original query by appending more related terms that reflect different user aspects. This process is called query expansion. Here our expansion method tries to select terms with various aspects underlying a query as much as possible. Our hypothesis is that, if the query covers more user aspects, the task of estimating the relevancy between the expanded query and documents will be more easier. The original query is expanded using the following three steps.

4.1.1 Candidate Extraction

Query reformulation with related query suggestions and completions are more effective for searching the most relevant documents that maximize the coverage [8, 21]. The query suggestions and completions from commercial search engines are employed as a resource to find the expansion terms. We retrieve query suggestions and query completions from major search engines (Google, Yahoo, and Bing) for a given query. Then the duplicates are filtered out after aggregating all suggestions and completions.

4.1.2 Generating Query Aspects

Multiple query suggestions and completions may contain candidates which reflect the same query aspect. Our observation on this aspect is that a group of candidates covers similar query aspect rather covering unique aspects. Table 1 depicts an example where we can see that multiple candidates represent the same aspect. In this table, we can see five aspect of query "grilling" including "grilling recipes", "grilling chicken", "grilling corn", "grilling lobster", and "grilling tips".

A soft clustering technique is then applied to the candidates based on frequent phrases to identify the query aspects. We make use of Lingo Clustering algorithm [22] to group the candidates into clusters. Some candidates may belong to more than one cluster. Then we used the cluster labels generated by the clustering algorithm as query aspects.

4.1.3 Selecting Expansion Terms

For a given query q, let assume that [L.sub.q] = {[l.sub.1], [l.sub.2], [l.sub.3], ...., [l.sub.K]} be the set of cluster labels generated by the previous section. We generate and select the expansion terms from these labels to expand the query. To select the expansion terms from the generated cluster labels, we introduce a expansion term selection algorithm. The pseudo-code of our expansion term selection (ETS) algorithm is as follows:

where [E.sub.t] denotes the set of expansion terms, l is the cluster label, and t is the term in label l. t [member of] l and t [??] q state that term t exists in l and t does not exist in q, respectively.

4.1.4 Query Expansion

Let [E.sub.t] = {[t.sub.1], [t.sub.2], [t.sub.3], ..., [t.sub.n]} be the set of selected terms for query q. We make use of these terms to expand the query. We append the selected terms with the query q and the expanded query is as like as follows:
[q.sub.exp] = q
for each term t; t [member of] [E.sub.t]  (6)
  [q.sub.exp] = [q.sub.exp] [union] t

where [q.sub.exp] denotes the expanded query.

4.2 Search Result Diversification

This section presents the diversification approach to re-rank the retrieval result for original query. The documents are re-ranked with respect to their relevancy between the expanded query [q.sub.exp] and the documents. We first extract multiple semantic and lexical features, then we apply a linear ranking function to rank the documents. Since the expanded query covers multiple query aspects, the re-ranked documents satisfy the diversity.

4.2.1 Feature Extraction

The lexical and semantic features are estimated using WordNet [23] and the content information of document and expanded query and the pre-trained word2vec (1) model on Google News Corpus. The lexical and semantic features are summarized in Table 2 and 3, respectively.

The notations in the Table 3 are defined as follows:

* In Meaningful POS (part-of-speech) percentage (MPP) feature [f.sub.MPP] (d), I(POS(t) [member of] M) returns 1 if the POS of term t belongs to the set M = {Noun, Verb, Adjective, Adverd} [24].

* In average concept similarity (ACS) feature [f.sub.ACS]([q.sub.exp], d), ConSim([t.sub.i], [t.sub.j]) returns the conceptual similarity between term [t.sub.i] and [t.sub.j] [24].

* In similarity (SIM) feature [f.sub.SIMw2v] ([q.sub.exp], d) based on Word2Vec, [mathematical expression not reproducible] and [mathematical expression not reproducible] denote the 300 dimensional vector representation of term ti and [t.sub.j] from pre-trained word2vec model, respectively.

We make use of MinMax normalization to normalize the features value into the range [0,1] as follows:

[bar.x] = [x - min(x)]/[max(x) - min(x)]

where x is the feature value and [bar.x] is the normalized feature value. min(x) and max(x) denote the minimum and maximum feature values of a specific feature, respectively.

4.2.2 Document Ranking

To re-rank the retrieved documents for original query, we estimate the document relevancy using a linear ranking approach considering all extracted features as follows:

[mathematical expression not reproducible] (7)

where [f.sub.i]([q.sub.exp], d) denotes the i-th feature and w[t.sub.i] denotes the feature importance. The higher the value of Rel([q.sub.exp], d), the higher the relevancy of the document d with the expanded query [q.sub.exp] is.


This section presents the details of the experiments and evaluation results of our proposed method on a standard dataset and compare with some known related methods.

5.1 Data Collection

We use the Web Track dataset from TREC 2012 [25]. There are 50 queries, each of which includes 3 to 8 subtopics identified by TREC assessors. All experiments are conducted on ClueWeb09 [26] collection. We used query suggestions and completions from Googe, Yahoo, and Bing search engine provided by NTCIR-10 English subtopic mining dataset [27]. The pre-trained Word2Vec model using Google News Corpus are employed to extract semantic features. To find the POS of each term, we make use of Stanford NLP Parser. We make use of Indri search engine [28] to retrieve top 500 documents from the clueweb09 collection.

5.2 Evaluation Metrics

Several metrics have been used in order to evaluate the diversification effectiveness of search engines. A good diversification system is the one that satisfies multiple information needs (or user intents) underlying a query that is submitted to that system by different users, or by the same user in different contexts. In the context of search result diversification, a query is represented by a set of subtopics or aspects (which generally correspond to user intents). The relevance of a document with respect to a query is judged separately for each subtopic, and is estimated by the ability of that document to cover different subtopics of the same query. In this research, we utilized three diversity metrics which are official in the diversity task of TREC Web track.

5.2.1 [alpha]-nDCG ([alpha]-normalized Discriminative Cumulative Gain)

[alpha]-nDCG@k [29] is computed as follows:

[[alpha] - nDCG@k] = [[alpha] - DCG@k]/[[alpha] - DCG'@k]

where [alpha] - DCG'@k is a normalization factor corresponding to the maximal value of [alpha] - DCG@k that gives the ideal document ranking. [alpha] - DCG@k is computed as follows:

[mathematical expression not reproducible]

where the parameter [alpha] ([alpha] [member of] [0,1]) represents the user satisfaction factor for the set of documents that have been already browsed by the user. This parameter ([alpha]) is generally fixed to 0.5. q is a query, S(q) is the set of subtopics underlying q, and [d.sub.i] (resp. [d.sub.j]) is the document ranked at the ith (resp. jth) position. rel(d, s) is a function that evaluates the relevance of a document d with respect to a given subtopic s. Note also that [alpha] - nDCG considers the set of already (k-1) selected documents when evaluating a document at position k. This means that the metric takes into account the dependency between the returned documents. Finally, note that [mathematical expression not reproducible] penalizes the coverage of already covered aspects of the query and [alpha] controls the amount of penalization.

5.2.2 ERR-IA (Expected Reciprocal Rank - Intent Aware)

ERR-IA(q,D) [30] for a given query q and over a set of returned documents D with respect to q is defined as follows:

[mathematical expression not reproducible]

where ERR(s,D) is the expected reciprocal rank and p(s|q) denotes the importance of subtopic s regarding to the query q (the more popular the subtopic s for q, the higher is p(s|q).

5.2.3 NRBP (Novelty and Rank-Biased Precision)

NRBP [31] is an extension of the RBP (Rank-Biased Precision) metric [32]. The basic intuition that NRBP uses is that, the user has some specific intent and is generally interested in one particular aspect of the query, at least at that time. NRBP is defined as follows:

[mathematical expression not reproducible]

where [d.sub.k] denotes the kth document, N is the (possible) number of aspects of a given query, J(d, i) = 1 if document d is relevant to the ith aspect of the query , and J(d, i) = 0 otherwise, C(k, i) is the number of documents at cut-off k that have been judged to be relevant to the ith aspect of the query, parameter [beta] [member of] [0,1] is used to model the patience level of the user, and parameter [alpha] [member of] [0,1] refers to the user declining interest.

In short, we use the above three official diversity evaluation metrics used in TREC Web Track.

5.3 Experimental Results

To measure the effectiveness of our aspect based query expansion method for SRD, we carried out experiments using different experimental settings. We retrieved top 500 documents for each query by using two baseline retrieval model, Language model with Jelinek-Mercer smoothing and Okapi BM25. We already described these to classical model in section 3. We used the parameter mu=2500, lambda=0.4 for Jelinek-Marcer smoothing and k1=1.2, b=0.75 and k3=7 for BM25 based method. We denote these two experimental settings as LM and BM25, respectively.

We carried out experiments using our reranking method presented on section 4.2 with documents retrieved by BM25. In setting QFLR, we re-ranked the retrieved documents by using a linear ranking method (i.e Eq. 7)with extracted features with respect to the original query. Then we expanded the query with the query suggestions' terms except stopwords and query terms. This setting is denoted by [Q.sub.n_exp]FLR. Then we applied our aspect-based query expansion method to expand the query and linear ranking with only lexical features to re-rank the documents in setting [Q.sub.a_exp]LFLR. Finally, in setting [Q.sub.a_exp]FLR, we used all features instead for lexical features to re-rank the documents with respect to the expanded query [q.sub.exp]. Table 4 summaries the description of all experimental settings.

The performance of our proposed methods, baseline, and some related [25] methods on TRECWeb track 2012 dataset in terms of three diversity metrics including ERR-IA, [alpha]-nDCG, and NRBP at the cut of 20 are reported in Table 5. Boldface indicates the best performance among all methods. We can see that, our two aspect-based query expansion methods [Q.sub.a_exp]LFLR and [Q.sub.a_exp]FLR performed better than all other methods including the normal query expansion based method [Q.sub.n_exp]FLR. Therefore, we can conclude that aspect-based query expansion can capture different query aspects which helps to increase the diversity of the ranking. Our proposed semantic features are not applied in setting [Q.sub.a_exp]LFLR whereas those are applied in setting [Q.sub.a_exp]FLR. The experimental results clearly demonstrate that our semantic features are effective to capture better relevancy.

The query-wise performance of our method in terms of the diversity metric [alpha]-nDCG@20 on TREC Web Track 2012 dataset is depicted in Fig. 2. The figure illustrates that the performance for each individual query are varied widely. We can see that, our method achieved more than 80% accuracy (i.e Query 05, Query 09, Query 41, etc.) for several queries . For an example query "porterville" (Query 09), our method achieved 85% accuracy. The aspect-based query expansion technique selected the expansion terms for this queries are: "recorder," "college," "ca," "school," "district," "fair," "weather," "police," "department," and "courthouse". These expansion terms are distinct from each other and all are related to the query. We can also see that each term is representing different user's query aspect for the query "porterville". The figure also conclude that our method failed for only two queries (Query 12 and Query 26). Considering the query, "dnr" (Query 12) which is an abbreviation query. The observation for this query is that our frequent phrase-based clustering algorithm was not good enough to generate meaningful cluster labels. In turns the expansion terms were not related to the abbreviation queries. We think this might be one of the plausible reasons for the failure. However, we can conclude from the experimental results on a benchmark dataset that our proposed aspectbased query expansion method and semantic features contributed to effectively diversify the documents. Our method also outperformed the baselines and known related methods in terms of all three standard diversity evaluation metrics.


This paper proposed an aspect-based query expansion method and multiple semantic features for search results diversification. To identify the query aspects, we applied a frequent phrase-based soft clustering technique to the query suggestions. Then we select the expansion terms from the cluster labels. We also proposed multiple semantic and lexical features to estimate the relevancy between the expanded query and the document. The experimental results on a benchmark TREC dataset clearly conclude that our proposed method is effective for search result diversification.

For future work, we have a plan to extract expansion terms from top retrieved web document for query expansion. Furthermore, it will be interesting to apply an aspect-based document diversification approach for results diversification.


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Md Shajalal (a*)

Masaki Aono (b*)

Muhammad Anwarul Azim (c*)

(*) Department of Computer Science and Engineering

(a) Bangladesh Army University of Science and Technology, Nilphamari, Bangladesh

(b) Toyohashi University of Technology, Toyohashi, Aichi, Japan

(c) University of Chittagong, Chittagong, Bangladesh

(1) word2vec (
Table 1. Multiple candidates reflect one aspect.

Query     Candidate                      Aspect

grilling  memorial day grilling recipes  Grilling Recipes
          easy grilling recipes
          grilling recipes
          grilling chicken               Grilling Chicken
          grilling chicken breasts
          grilling chicken leg
          grilling corn on the cob       Grilling Corn
          grilling corn
          grilling lobster tails         Grilling Lobster
          grilling lobster
          grilling tips                  Grilling Tips
          outdoor grilling tips
          perfect grilling tips

Table 2. lexical features

Type      Features                     Description

Lexical   01. [f.sub.LS]               Lexical similarity based on
Features  ([q.sub.exp], d)             edit distance
          02. [f.sub.TO]               % of overlapping query terms
          ([q.sub.exp], d)
          03. [f.sub.SynO]             % of overlapping synonym of
          ([q.sub.exp], d)             query terms.
          04. [f.sub.BR](q, d)         Baseline rank of each
                                       individual document
          05. [f.sub.VisTerm(d)]       Number of visible terms on
                                       the page
          06. [f.sub.TTerm](title(d))  Number of terms in the page
          07. [f.sub.avgTL](d)         Avg. length of visible terms
                                       on the document
          08. [f.sub.fracAT](d)        Fraction of anchor text on
                                       the document
          09. [f.sub.fracVT](d)        Fraction of visible text on
                                       the document
          10. [f.sub.fracS](d)         Stopword and Non-stopword

Table 4. Summary of all experimental settings.

Run                Description

LM                 Documents retrieved using language model
BM25               Documents retrieved with BM25 retrieval model
QFLR               Original query and linear ranking with all features
[Q.sub.n_exp]FLR   Query expansion with suggestions's terms and linear
                   ranking with all features
[Q.sub.a_exp]LFLR  Aspect-based query expansion and linear ranking with
                   lexical features
[Q.sub.a_exp]FLR   Aspect-based query expansion and linear ranking with
                   lexical and semantic features

Table 5. Experimental results of our method, baseline and some known
related methods on TREC Web Track 2012 in terms of ERR-IA, [alpha]-nDCG,
and NRBP at cut of 20. Boldface indicates the best performance ammong

Type        Method             ERR-IA@20  [alpha]-nDCG@20  NRBP

Baseline    BM25 [19]          0.2253     0.3105           0.1738
Retrieval   LM [16]            0.157889   0.2143           0.1237
Our Method  [Q.sub.a_exp]FLR   0.3447     0.4438           0.3033
            [Q.sub.a_exp]LFLR  0.3015     0.3925           0.2685
            [Q.sub.n_exp]FLR   0.2475     0.3475           0.2348
            QFLR               0.2354     0.3257           0.1925
Related     ICTNET [25]        0.326      0.422            0.280
Methods     udel [25]          0.325      0.419            0.282
            LIA [25]           0.318      0.424            0.268
            udel fang [25]     0.300      0.420            0.241
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Author:Shajalal, Md; Aono, Masaki; Anwarul Azim, Muhammad
Publication:International Journal of New Computer Architectures and Their Applications
Article Type:Report
Date:Apr 1, 2018
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