FEATURE SELECTION-MODEL-BASED CONTENT ANALYSIS FOR COMBATING WEB SPAM

David C. Wyld et al. (Eds) : CSITY, SIGPRO, AIFZ, NWCOM, DTMN, GRAPHHOC - 2016
pp. 27–34, 2016. © CS & IT-CSCP 2016 DOI : 10.5121/csit.2016.60403
FEATURE SELECTION-MODEL-BASED
CONTENT ANALYSIS FOR COMBATING
WEB SPAM
Shipra Mittal1*
and Akanksha Juneja2
Department of Computer Science & Engineering,
National Institute of Technology, Delhi, India
*Corresponding Author at - Department of Computer Science & Engineering,
National Institute of Technology, Delhi, India
1
mittal.shipra90@gmail.com,
2
akankshajuneja@nitdelhi.ac.in
ABSTRACT
With the increasing growth of Internet and World Wide Web, information retrieval (IR) has
attracted much attention in recent years. Quick, accurate and quality information mining is the
core concern of successful search companies. Likewise, spammers try to manipulate IR system
to fulfil their stealthy needs. Spamdexing, (also known as web spamming) is one of the
spamming techniques of adversarial IR, allowing users to exploit ranking of specific documents
in search engine result page (SERP). Spammers take advantage of different features of web
indexing system for notorious motives. Suitable machine learning approaches can be useful in
analysis of spam patterns and automated detection of spam. This paper examines content based
features of web documents and discusses the potential of feature selection (FS) in upcoming
studies to combat web spam. The objective of feature selection is to select the salient features to
improve prediction performance and to understand the underlying data generation techniques.
A publically available web data set namely WEBSPAM - UK2007 is used for all evaluations.
KEYWORDS
Web Spamming, Spamdexing, Content Spam, Feature Selection & Adversarial IR
1. INTRODUCTION
As the scope of web grows beyond limits, it is more prone to profanation. From accessing
information to interacting and connecting with people, from e-commerce to e-businesses, Internet
covers almost each and every aspect of our lives. It helps in bringing new opportunities to
people. According to Sam Lucero, analyst at ABI Research in Oyster Bay, "anything intelligent
would have an online presence” [1]. But, as it is said, every massive technology has its own
benefits and challenges, same is the case with Internet and World Wide Web. Accurate and
quality information retrieval is one of those major challenges. As business vendors recognize the
value of web for reaching out to millions of customers, they try to gain high visibility for their
websites on search engine result page (SERP). This rising need to rank highly in search results in
order to recognize among web users, gives birth to the term web spamming (or, spamdexing) [2].
Spamdexing, as the name implies, takes advantage of web indexing system, allowing spammers
to deceive search engine (SE) ranking of specific documents.

28 Computer Science & Information Technology (CS & IT)
Ranking system of SEs involves various content-based and graph-based measures. Spammers
exploit these parameters to artificially inflate the ranking of web documents. Spam techniques
range from stuffing a page with large number of authority references or popular query keywords,
thereby causing the page to rank higher for those queries, to setting up a network of pages that
mutually reinforce their page value to increase the score of some target pages or the overall
group.
Recently [3; 4], all major SEs such as Google, Yahoo etc. have identified web spam as a tangible
issue in IR process. It not only deteriorates the search quality but also cause wastage of
computational and storage resources of a SE provider. A financial loss of $50 billion was caused
due to spam in the year 2005 [5]. In the year 2009, it was estimated at $130 billion [6]. Further, it
weakens people’s trust and might deprive legitimate websites of user’s visibility and revenue.
Therefore, identifying and combating spam becomes a top priority for SE providers.
According to web spam taxonomy presented in the work of Spirin and Han [7], web spam is
broadly classified into four categories namely content spam [8], link spam [9; 10], cloaking and
redirection [11; 12], and click spam [13]. This research work primarily focuses on the detection
of content spam which is the most common and frequently occurring spam [14].
IR systems examine the content of pages in the corpus to retrieve the most relevant document
with respect to a specific search query. “Term Frequency-Inverse Document Frequency” (TF-
IDF) or another similar approach is used to access the “most similar” (relevant) documents to the
search query. In TFIDF, “the relevance of the search terms to the documents in corpus is
proportional to the number of times the term appeared in the document and inversely proportional
to the number of documents containing the term.” Spammers exploit Term Frequency (TF)
scoring by overstuffing content fields (title, body, anchor text, URL etc.) of a page with a number
of popular search terms so as to boost its relevancy score for any search query. It can be measured
as:
where q refers to query, p denotes a web page in the corpus, and t denotes the term.
Machine learning is a field of study that deals with automated learning of patterns, within the data
belonging to different classes or groups, with an aim to differentiate between the classes or
groups. An effective machine learning algorithm is expected to make accurate predictions about
categorization of unseen data based on the learnt patterns. Specifically, supervised machine
learning involves predicting the class of an unseen (new) data sample based on the decision
model learnt using the existing (training) data. Therefore, knowledge of machine learning may be
appropriately utilized for web spam detection.
Several machine learning methods to combat content spam were introduced in the past researches
of adversarial IR and web spam domain. Egele et al. [15] examined the importance of different
text and link metrics in web ranking system and utilize C4.5 supervised learning algorithm to
remove spam links. They deployed their own mechanism to generate data to carry out the
experiment.
Ntoulas et al. [16] presented an approach for detecting spam based on content analysis. They
extracted several content features and presented a comprehensive study about the influence of
these features in web spam domain.

Computer Science & Information Technology (CS & IT) 29
Prieto et.al [17] suggested a number of heuristics to identify all possible kinds of web spam and
developed a system called SAAD (Spam Analyzer and Detector) for efficient web spam
detection. The beneficial trait is that the system was tested on different data sets and proved to be
effective for more accurate classification of spam pages.
Araujo and Romo [18] proposed an interesting approach of spam detection by comparing the
language models (LM) [19] for anchor text and pages linked from these anchor text. KL-
divergence [20] was used to measure the discrepancy between two LMs.
However, spammers are continuously adapting themselves to circumvent these barriers. This
research work presents a content-based spam detection approach using supervised learning. The
aim of this study is to draw a clear understanding of underlying process of web spamming by
examining already extracted features. Moreover, the work aims at selecting salient features from
the existing ones to stimulate further studies in the domain of “adversarial information retrieval
[21]”. A filter based feature selection technique is employed to uncover important patterns in
order to classify websites as spam or ham (non-spam). The proposed method is observed to be
efficient in terms of both computational complexity and classification accuracy.
The rest of the paper organizes as follows. A general methodology of applying feature selection
technique for adversarial classification is presented in section 2. Finally, experimental results are
shown in section 3 and section 4 concludes the paper.
2. METHODOLOGY
2.1 Experimental Data Set
Widely known web spam data set WEBSPAM-UK2007 [22] is used to carry out the experiments.
This dataset was released by Yahoo especially for Search Engine Spam project. The data set was
also used for “Web Spam Challenge 2008”. It is the biggest known web spam data set having
more that 100 M web pages from 114,529 hosts. However, only 6,479 hosts were manually
labelled as spam, non-spam and undecided. Among these, approx 6% hosts are spam, i.e., data is
imbalanced in nature. The data set consist a separate training and testing data set, but we
combined the two sets together to evaluate our model since the percentage of spam hosts were
small in both of them. Further, we neglect the hosts labelled as “undecided” and conduct our
experiment for a group of 5,797 hosts. The training set was released with pre-extracted content
feature set which are examined in this study to select salient and optimal features.
Existing content- based heuristics for detecting web spam
The content feature set proposed by Ntoulas et al. [16] comprises of 98 features based on
following heuristics:
• Number of words in the page: “Term Stuffing” is a common spamming technique to
increase visibility of a web document on typical queries or search terms. Sometimes the
proportion of common terms in a page is very high. Therefore, authors suggest counting
number of words per page. Very large value of the proposed heuristic indicates the strains
of spam in the page.
• Number of words in the title: Many times, a page title is stuffed with unrelated keywords
and terms because of its high weightage in search engines text metrics. As a spam
detection method, authors propose measure of number of words in the title
• Average word length of the document: In order to combat composite keywords
spamming, authors propose to measure the average word length

• Fraction of anchor text in the page: Due to the fact that “anchors” are used to describe the
association between two linked pages, spammers misuse them to create false associations
• Ratio of visible text: The authors propose this heuristic to detect “hidden content” in a
page
• Compression Ratio: The proposed features helps in determining the level of redundancy
in the page
• Independent and Dependent LH: These techniques utilize the independent and dependent
n-grams probability to detect spam. More precisely, content of each page is divided into
n-g of n consecutive words to calculate the probability of document by individual n-g
probabilities. However, this feature is computationally expensive.
The performance analysis of these features and its comparison after applying feature selection
techniques is presented in section 3.
2.2 Proposed Methodology
This research work focuses on the contribution of feature selection in adversarial IR applications.
The common issues in the spam domain are listed as: small sizes of samples, unbalanced data set
and large input dimensionality due to several pages in a single web document. To deal with such
problems, a variety of feature selection methods have been designed by researchers in machine
learning and pattern recognition. This work employs univariate filter feature selection to improve
the prediction performance of decision model. The existing heuristics on which feature selection
is performed are already discussed in previous subsection. The idea of applying feature selection
in the existing features is two-fold: these features were utilized in many previous studies [16; 17;
18; 23] for effective spam detection; the heuristics recognized as baseline for further studies in
the underlying domain.
Due to aforementioned reasons, it can be expected that, for spam documents classification,
feature selection techniques will be of practical use for later researches in information retrieval
and web spam.
2.2.1 Methods for Web Spam Detection
This section describes the algorithms and methods used for evaluation of this work.
Classification Algorithm
For the appropriate prediction, we have tried various classification methods, namely, k-nearest
neighbour, linear discriminant analysis, and support vector machine (SVM). As per experiments,
SVM is observed to achieve better results for binary classification in comparison to other
classifiers. Therefore, this research work SVM is utilised to learn decision model.
SVM [24] classify objects by mapping them to higher dimensional space. In this space, SVM
train to find the optimal hyperplane, i.e., the one with the maximum margin from the support
vectors (nearest patterns).
Consider a training vector we define a vector y such that
decision function can be defined as:

The weight vector w is defined as:
where is the Lagrange’s multiplier used to find the hyperplane with maximum distance from
the nearest patterns. Patterns for which are support vectors.
The possible choice of decision function when data points are not separable linearly can be
expressed as:
where, and C is the penalty parameter. Value of C =10 is used for
experimental evaluation.
Performance Evaluation
In order to evaluate the model, 10-fold cross validation technique is used. The results are shown
in terms of classification accuracy, sensitivity, and specificity, whose values are obtained by
analysing the cost matrix [25].
Sensitivity can be defined as the number of actual positive instances (spam) that are correctly
predicted as positive by the classifier. Conversely, specificity determines proportion of actual
negatives (non-spam hosts) that are correctly classified. Accuracy can be defined as total number
of instances that are correctly predicted by the classifier.
Univariate Filter Feature Selection: Simple yet efficient
In order to improve the performance of SVM, feature selection is implemented. Univariate filter
based feature selection has been utilised due to the fact that it is computationally simple, fast,
efficient, and inexpensive. In filter based feature selection, “features are selected independently to
induction algorithm” [26; 27; 28]. The measurement for feature selection is chosen as Mutual
Information Maximization (MIM) [29]. It is a simple method to estimate rank of features based
on mutual information. Mutual information is defined as a relevancy measure, determining how
relevant a feature is for corresponding targets. The criterion can be expressed as follows:
where is the nth
feature of training matrix X, c is the class label and refers to mutual
information between the two. Mutual Information between two random variables p and q can be
determined as:
3. EXPERIMENTAL RESULTS AND DISCUSSION
Table 1 shows the performance of SVM on existing feature set whereas Table 2 shows the
prediction performance after feature selection. It is clearly visible that feature selection technique
on precompiled measures outperforms the performance of complete feature set. The results show
a significant gain in classifier’s accuracy in terms of both valuation measures (i.e., specificity and
sensitivity). Approximately 3% increase in specificity and 2% increment in accuracy and
sensitivity is reported.

Table 1: Performance of content- based feature sets using SVM
Feature Set Performance Measure (in percentage)
(98 features) Accuracy Sensitivity Specificity
Content 79.9 61.8 79.2
Table 2: Performance of content- based feature sets after feature selection using SVM
Filter based Feature
Selection
Number of features
Top
10
Top
20
Top
30
Top
40
Top
50
Top
60
Top
70
Top
80
Top
90
Performance
Measure (in
percentage)
Accuracy 76.7 78.6 81.6 81.6 81.4 81.1 82.1 80.8 80.7
Sensitivity 46.6 51.2 51.3 55.3 55.3 57.1 63.1 63.1 62.7
Specificity 81.5 81.6 81.6 82.6 82.7 82.7 82.9 80.8 80.7
4. CONCLUSIONS AND FUTURE PERSPECTIV
In this study, we take into account existing heuristics for detecting spam by means of content
analysis. This experiment compares the performance results of pre-determined features with the
performance of features achieved after feature selection. The experimental results demonstrate
that classifier performance increases with reduced (reduced from 98 features to 70 features) set of
salient features. Furthermore, we believe that feature selection undermines the inherent risk of
imprecision and over-fitting caused due to unbalanced nature of dataset. However, a robust and
optimal feature selection model is still a need to uncover.
Multivariate feature selection and wrapper based feature selection can be addressed as a
prominent future study in web spam community. A second line of future research will be
extension of heuristics extracted using both content analysis and web graph mining. Other
interesting opportunities oriented towards different machine learning approaches such as fuzzy
logic, neural network etc. Since, there is no clear separation between spam and ham pages, i.e.,
definition of spam may be vary from one person to another, use of fuzzy logic can be seen as a
promising line of future work in detection of web spam.
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AUTHORS
Ms. Shipra is currently pursuing her Masters in Analytics (Computer Science and
Technology) from National Institute of Technology (NIT), New Delhi, India.Prior to
this she has received her B.Tech degree in Computer Science and Engineering and has
more than 1 year of work experience in Digital Marketing. Her current area of research
is machine learning and pattern recognition problems domains of adversarial
information retrieval and search engine spam.
Ms. Akanksha Juneja is currently working as Assistant Professor in Department of
Computer Science and Engineering, National Institute of Technology Delhi. She is also
a PhD scholar at School of Computer & Systems Sciences (SC&SS), Jawaharlal Nehru
University (JN U), New Delhi, India. Prior to this she has received her M.Tech degree
(Computer Science & Technology) from SC&SS, JNU, New Delhi. Her current area of
research is machine learning and pattern recognition problems domains of image
processing and security.

FEATURE SELECTION-MODEL-BASED CONTENT ANALYSIS FOR COMBATING WEB SPAM

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