authorship attribution using probabilistic context-free grammars

Authorship Attribution Using Probabilistic Context-Free Grammars

Sindhu Raghavan, Adriana Kovashka, Raymond MooneyThe University of Texas at Austin

Authorship Attribution

• Task of identifying the author of a document

• Applications– Forensics (Luckyx and Daelemans, 2008)

– Cyber crime investigation (Zheng et al., 2009)

– Automatic plagiarism detection (Stamatatos, 2009)

– The Federalist papers study (Monsteller and Wallace, 1984)– The Federalist papers are a set of essays of the US constitution– Authorship of these papers were unknown at the time of publication– Statistical analysis was used to find the authors of these documents

Existing Approaches

• Style markers (function words) as features for classification (Monsteller and Wallace, 1984; Burrows, 1987; Holmes and Forsyth, 1995; Joachims, 1998; Binongo and Smith, 1999; Stamatatos et al., 1999; Diederich et al., 2000; Luyckx and Daelemans, 2008)

• Character-level n-grams (Peng et al., 2003)

• Syntactic features from parse trees (Baayen et al., 1996)

• Limitations– Capture mostly lexical information– Do not necessarily capture the author’s syntactic style

Our Approach• Using probabilistic context-free grammar (PCFG)

to capture the syntactic style of the author

• Construct a PCFG based on the documents written by the author and use it as a language model for classification– Requires annotated parse trees of the documents

How do we obtain these annotated parse trees?

Algorithm – Step 1

Treebank each document using a statistical parser trained on a generic corpus– Stanford parser (Klein and Manning, 2003)

– WSJ or Brown corpus from Penn Treebank (http://www.cis.upenn.edu/~treebank)

…………………..….……..

Training documents

Alice Bob Mary John

S NP VP .8S VP .2NP Det A N .4NP NP PP .35NP PropN .25

Probabilistic Context-Free Grammars

Train a PCFG for each author using the treebanked documents from Step 1

Alice Bob Mary John

Test document

Multiply the probability of the top parse for each sentence in the test document

Test document

Multiply the probability of the top parse for each sentence in the test document

Label for the test document

Experimental Evaluation

DataData set # Authors Approx #

Words/authorApprox #

Sentences/author

Football 3 14374 786

Business 6 11215 543

Travel 4 23765 1086

Cricket 4 23357 1189

Poetry 6 7261 329

Blue – News articles Red – Literary worksData sets available at www.cs.utexas.edu/users/sindhu/acl2010

Methodology

• Bag-of-words model (baseline)

– Naïve Bayes, MaxEnt• N-gram models (baseline)

– N=1,2,3

• Basic PCFG model• PCFG-I (Interpolation)

Methodology

• Bag-of-words model (baseline)

– Naïve Bayes, MaxEnt• N-gram models (baseline)

– N=1,2,3

• Basic PCFG model• PCFG-I (Interpolation)

Basic PCFG

• Train PCFG based only on the documents written by the author

• Poor performance when few documents are available for training– Increase the number of documents in the training set– Forensics - Do not always have access to a number of

documents written by the same author– Need for alternate techniques when few documents are

available for training

PCFG-I

• Uses the method of interpolation for smoothing

• Augment the training data by adding sections of WSJ/Brown corpus

• Up-sample data for the author

Results

Performance of Baseline Models

Inconsistent performance for baseline models – the same model does not necessarily perform poorly on all data sets

Performance of PCFG and PCFG-I

PCFG-I performs better than the basic PCFG model on most data sets

PCFG Models vs. Baseline Models

Best PCFG model outperforms the worst baseline for all data sets, but does not outperform the best baseline for all data sets

PCFG-E• PCFG models do not always outperform N-gram

models

• Lexical features from N-gram models useful for distinguishing between authors

• PCFG-E (Ensemble) – PCFG-I (best PCFG model)

– Bigram model (best N-gram model) – MaxEnt based bag-of-words (discriminative classifier)

Performance of PCFG-E

21PCFG-E outperforms or matches with the best baseline on all data sets

Significance of PCFG

22Drop in performance on removing PCFG-I from PCFG-E on most data sets

(PCFG-E – PCFG-I)

Conclusions

• PCFGs are useful for capturing the author’s syntactic style

• Novel approach for authorship attribution using PCFGs

• Both syntactic and lexical information is necessary to capture author’s writing style

Thank You

authorship attribution using probabilistic context-free grammars

np np pp

np det

s np vp

np propn

characterlevel ngrams

test documentalgorithm

parse trees baayen

syntactic features

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