Paraphrase Acquisition via Crowdsourcing and Machine Learning
Steven Burrows, Martin Potthast, and Benno Stein

To paraphrase means to rewrite content whilst preserving the original meaning. Paraphrasing is important in fields such as text reuse in journalism, anonymising work, and improving the quality of customer-written reviews. This paper contributes to paraphrase acquisition and focuses on two aspects that are not addressed by current research: (1) acquisition via crowdsourcing, and (2) acquisition of passage-level samples. The challenge of the first aspect is automatic quality assurance; without such a means the crowdsourcing paradigm is not effective, and without crowdsourcing the creation of test corpora is unacceptably expensive for realistic order of magnitudes. The second aspect addresses the deficit that most of the previous work in generating and evaluating paraphrases has been conducted using sentence-level paraphrases or shorter; these short-sample analyses are limited in terms of application to plagiarism detection, for example. We present the Webis Crowd Paraphrase Corpus 2011 (Webis-CPC-11), which recently formed part of the PAN 2010 international plagiarism detection competition. This corpus comprises passage-level paraphrases with 4 067 positive samples and 3 792 negative samples that failed our criteria, using Amazon's Mechanical Turk for crowdsourcing. In this paper, we review the lessons learned at PAN 2010, and explain in detail the method used to construct the corpus. The empirical contributions include machine learning experiments to explore if passage-level paraphrases can be identified in a two-class classification problem using paraphrase similarity features, and we find that a k-nearest-neighbor classifier can correctly distinguish between paraphrased and non-paraphrased samples with 0.980 precision at 0.523 recall. This result implies that just under half of our samples must be discarded (remaining 0.477 fraction), but our cost-analysis shows that the automation we introduce results in a 18% financial saving and over 100 hours of time returned to the researchers when repeating a similar corpus design. On the other hand, when building an unrelated corpus requiring say 25% training data for the automated component, we show that the financial outcome is cost-neutral, whilst still returning over 70 hours of time to the researchers. The work presented here is the first to join the paraphrasing and plagiarism communities.