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Web Spam

Seminar: Future Of Web Search

Know Your Neighbors: Web Spam

Detection using the Web Topology

Presenter: Sadia Masood

Tutor : Klaus Berberich

Date : 17-Jan-2008

University of Saarland


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Web Spam

The Agenda

Focus of the First Paper

Motivation

The Objective

DATASET

Link Based Features

Content Based Features

Using the Content & Link Based Features

Using the Web Topology

Conclusion

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Web Spam

The Agenda


Motivation

The Objective

DATASET

Link Based Features




Conclusion

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What is Web Spamming?

Any deliberate human action that is meant to trigger an

unjustifiably favorable relevance or importance of some web page

considering the pages true value [5]

Also called Search Engine SpammingorSpamdexing

[5]: Z. Gyongyi and H. Garcia-Molina. Web spam taxonomy. In First International Workshop on

Adversarial Information Retrieval on the Web, 2005.5


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What is Web Spamming?

Example

Query: Kaiser Pharmacy (at the time paper was written 2005)

Result: had techdictionary.com as a 3rd hit

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Why is Web Spamming Bad?

Search Engines suffer because

It damages the search engines reputation

there is a the cost involved to crawling, indexing, storing the spam

pages.

Users suffer because

precision in the query results is lowered

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How is it done?

Web Spamming Techniques involve

Boosting Techniques

Content spam (e.g, Term repetition)

Link spam (e.g Link farming)

Hiding Techniques

For example, Content hiding, Cloaking

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What do the Search Engines Ultimately want ?

Want to calculate and return the exact page ranks based on

relevance and importance

Want to avoid the spam pages altogether before they use

resources that might be used in storing or processing those webpages

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Web Spam

The Agenda


Motivation

The Objective

DATASET

Link Based Features




Conclusion

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The Flow

Feature extraction

Combining link & content based features

Link based

Content based

Classification

Clustering Propagation Stacked graphical learning

Smoothing

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Web Spam

The Agenda


Motivation

The Objective

DATASET

Link Based Features




Conclusion

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Web Spam

Data Set

Data Set

Collected in May 2006

Publicly available WEBSPAM-UK2006 dataset [15]

Pages from .uk domain

77.9 million pages were collected corresponding to 11,400 hosts

A group of volunteers labeled them normal, borderline or spam

6,552 hosts evaluated

[15] http://www.yr-bcn.es/webspam/datasets/14


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Data Set

Distribution of host labels, as judged by human volunteers

Evaluation

True positive rate, or Recall R False positive rate

F-measure

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Web Spam

The Agenda


Motivation

The Objective DATASET

Link Based Features




Conclusion

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Link Based Features

Degree related measures

PageRank

TrustRank Truncated PageRank

Estimation of d supporters

140 features per host

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Link Based Features

Page Rank

Uses link structure to determine importance or popularity of a web page

Intuition:

A web page is important if several other important pages point to

it

PageRank of a page influences and is being influenced by

importance of the other pages

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Link Based Features

Page Rank

Calculation: Initial scores already defined for all the pages

A

B

C

D

[6] http://en.wikipedia.org/wiki/PageRank

A

B

C

D

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Link Based Features

Page Rank

Page Rank with Random Surfer Model

d is damping factor,

M(pi) is the set of pages that link topi,

L(pj) is the number of outbound links on pagepj,

Nis the total number of pages.

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Li k B d F t


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Link Based Features

Trust Rank

A page having high pageRank is more likely to be spam if it had no

relationship with a set of trusted pages

How it works?

Determine the seed set S (using high PageRank or high out-degree)

Determine good or bad nodes of the set S (using oracle)

E.g, S ={ , , }

where, good =

bad =21

Li k B d F t


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Link Based Features

Trust Rank

Image [4]: http://www.tejedoresdelweb.com/slides/2007_ojobuscador_madrid_spam.pdf

Calculate & propagate the trust from good pages (adjusting trust

attenuation)

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Link Based Feat res


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Link Based Features

Histogram of ratio b/w TrustRank & PageRank

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Link Based Features


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Link Based Features

Truncated Page Rank

A variant of PageRank, diminishes the influence of a page

to the PageRank score of its close neighbors

spamweb web

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Link Based Features


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Link Based Features

Estimation of d-supporters

x is d-supporter of node y if shortest path from x to y has

length d Nd(x) be the set of d-supporters of page x

For each page x, cardinality of Nd(x) is an increasing fuction

with respect to d.

Image [4]25

Link Based Features


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Link Based Features

Bottleneck Number

The bottleneck measure for page x, defined as

indicates the minimum rate of growth of neighbors of x up to a

certain distance

spam pages have smaller bottle neck numbers than non-spam

pages

Bottleneck: Non-Spam & Spam

Image [4] 26

Link Based Features


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Link Based Features

Histogram of b4(x) of Spam and Non-spam Pages

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The Agenda


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Web Spam

The Agenda


Motivation

The Objective

DATASET

Link Based Features

Content Based Features Using the Content & Link Based Features


Conclusion

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Number of words in the page, title & average word length

Fraction of anchor text

Fraction of visible text

Compression rate

Corpus precision & corpus recall

Query precision and query recall Independent trigram likelihood

Entropy of trigrams

96 features per host

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Co te t ased eatu es

Average word length

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Compression Rate

Compression rate = size of compressed text (visible text)

size of uncompressed text

Precision and Recall

F= Frequent terms in the collection

T= Terms in the page

Q= Frequent terms in the query log

Corpus Recall = | F T | / | F |

Query Recall = | Q T | / | Q |

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Corpus Precision = | F T | / | T |

Query Precision = | Q T | / | T |

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Entropy of trigrams (Compression)

Calculated on distribution of trigrams

Let

be probability distribution on trigrams of a page

be the set of all trigrams in a page

Entropy of trigrams=

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The Agenda


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Web Spam


Motivation

The Objective DATASET

Link Based Features


Using the Link and Content Features


Conclusion

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Using Link and Content Based Features


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Cost Sensitive Decision Tree

Bagging

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Using Content & Link Based Features


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Comparing Link and Content based features

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The Agenda


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Web Spam


Motivation

The Objective

DATASET

Link Based Features




Conclusion

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Observation :

Similar pages tend to be linked to be linked together more frequently

than dissimilar ones

Similar pages tend to be linked to be linked together more frequentlythan dissimilar ones

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Using the Web Topology -Topological Dependencies of Spam Nodes


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Sin

Sout= No. of spam hosts linked by x

All hosts linked by x

Sout

Sin= No. of spam hosts linked to xAll hosts linked to x

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Clustering

Using METIS graph clustering algorithm

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Clustering

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Use the graph topology to smooth predictions by propagating them

as random walks

Main Idea

Use thepredicted spamicityof a particular classification method and

start a random walk with the restart probabilty 1-

Where

h: host

p(h) : [0..1] ( p(h)=0: non-spam , p(h)=1: spam, for each host h)

v(0) : vector such that v(0)h =

outdeg(g): the out-degree of g

Propagation

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Using the Web Topology- Propagation


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Applied with three forms of random walk

on the host graph (forward)

on the transposed host graph (backward)

on the undirected host graph (both)

Observed improvements on out of other tried values

Propagation

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Results with in the table

Propagation

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Stacked Graph Learning

Uses initial predictions by the classification scheme C for

all the objects in the data set

Generates a set of extra features for each object based on

relevance ( w.r.t in-links, outlinks or both)

Let r(h) be the set if pages related to host h

Then,

Adds these extra features f(h) to the input of C and run the algorithm

again

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Improvement is seen in the first Iteration

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Second Iteration showed even better results

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The Agenda


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Web Spam


Motivation

The Objective

DATASET

Link Based Features

Content Based Features Using the Web Topology

Conclusion

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Conclusion


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Experiments have clearly shown that

Combining both the link and content based features bringssignificant improvement to spam detection techniques

Using web graph dependencies , we can detect the web spam by

turning spammers ingenuity against themselves

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Thank You!

Q&A and Discussion

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References-1


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[1] www.milliondollarhomepage.com

[2] http://www.dcc.uchile.cl/~ccastill/papers/cdgms_2006_know_your_neighbors.pdf

[3] http://images.google.com/imgres?

[4] http://www.tejedoresdelweb.com/slides/2007_ojobuscador_madrid_spam.pdf

[5] Z. Gyongyi and H. Garcia-Molina. Web spam taxonomy. In First International

Workshop on Adversarial Information Retrieval on the Web, 2005.

[6] http://en.wikipedia.org/wiki/PageRank

[7] L. Becchetti, C. Castillo, D. Donato, S. Leonardi, and R. Baeza-Yates. Link-based

characterization and detection of Web Spam. In AIRWeb, 2006.

[8] http://infolab.stanford.edu/~zoltan/publications/gyongyi2006link.pdf52

References-2
http://www.milliondollarhomepage.com/http://www.dcc.uchile.cl/~ccastill/papers/cdgms_2006_know_your_neighbors.pdfhttp://images.google.com/imgreshttp://www.tejedoresdelweb.com/slides/2007_ojobuscador_madrid_spam.pdfhttp://infolab.stanford.edu/~zoltan/publications/gyongyi2006link.pdfhttp://infolab.stanford.edu/~zoltan/publications/gyongyi2006link.pdfhttp://www.tejedoresdelweb.com/slides/2007_ojobuscador_madrid_spam.pdfhttp://images.google.com/imgreshttp://www.dcc.uchile.cl/~ccastill/papers/cdgms_2006_know_your_neighbors.pdfhttp://www.milliondollarhomepage.com/


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[9] L. Becchetti, C. Castillo, D. Donato, S. Leonardi, and R. Baeza-Yates. Using

rank propagation and probabilistic counting for link-based spam detection.

Technical report, DELIS Dynamically Evolving, Large-Scale Information

Systems, 2006.

[10] http://en.wikipedia.org/wiki/Cross-validation

[11]A. Ntoulas, M. Najork, M. Manasse, and D. Fetterly. Detecting spam web

pages through content analysis. In Proceedings of the World Wide Web

conference, pages 8392, Edinburgh, Scotland, May 2006.[12] http://www.salford-systems.com/doc/BAGGING_PREDICTORS.pdf

[13] Z. Gyongyi, H. Garcia-Molina, and J. Pedersen. Combating web spam with

TrustRank. In Proc. of the 30th VLDB Conf., 2004.

[14] M. Henzinger, R. Motwani, and C. Silverstein. Challenges in web search

engines. ACM SIGIR Forum, 36(2), 2002.

[15] http://www.yr-bcn.es/webspam/datasets/

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http://en.wikipedia.org/wiki/Cross-validationhttp://www.salford-systems.com/doc/BAGGING_PREDICTORS.pdfhttp://www.salford-systems.com/doc/BAGGING_PREDICTORS.pdfhttp://en.wikipedia.org/wiki/Cross-validation

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