Dellert, Johannes

Rehm, Georg

Witt, Andreas

Zinsmeister, Heike

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Corpus Masking: Legally Bypassing LicensingRestrictions for the Free Distribution of TextCollectionsGeorg Rehm, Andreas Witt, Heike Zinsmeister,and Johannes Dellert1 IntroductionThough XML-annotated text collections are commonplace inhumanities computing, the value of the annotation is oftenunderestimated, as interesting applications can be realised byignoring the content and considering the annotation exclusively.At the same time, the distribution of text collections (e. g.,linguistic resources) is often restricted by rigid licenceagreements. Usually, a corpus consists of a source textcollection (STC) acquired from third parties such as web sitesor publishers, and annotation layers that refer to, for example,structural or linguistic properties. In practically all cases theSTC is a copyrighted property, so that it is up to the copyrightholder to decide if, and under which conditions, the corpus - acrucial part of which is the STC - can be made available to thepublic or to the research community.The example we use in this paper is TüBa-D/Z ("TübingenTreebank of Written German" (Telljohann et al, 2004 &Telljohann et al, 2006)). This manually annotated treebank isbased on a CD ROM that contains an archive of the issues thenewspaper die tageszeitung (taz) has published since 1986. Ifa researcher (the licencee) wants to obtain TüBa-D/Z, availablefor academic purposes free of charge, he or she has to sign alicence agreement with Tübingen University's LinguisticsDepartment (the licencer) which states that the licencer is thecopyright holder of the annotation and that the STC, aspublished on the taz CD ROM, is copyrighted by contrapressPage 166Published in: Schmidt, Sara/Siemens, Ray/Kumar, Amit/Unsworth, John (eds.): Digital Humanities 2007.Conference Abstracts - Urbana-Champaign: University of Illinois, 2007. pp. 166-170.media GmbH. The licencee has to certify that he, she or theinstitution the person works for has a valid licence for this CDROM.1Figure 1: Masking linguistic corpora by example of the TüBa-D/Z treebankWe propose the notion of corpus masking, i. e., obfuscating theSTC, but not the annotation layer(s), the STC is "removed", sothat the original licensing restrictions no longer hold for the"new" resource. The advantage is that the valuable annotationinformation can be made available for free (see figure 1).22 Corpora – Licence Restrictions – SustainabilityWhen linguists have created a corpus it can become quitedifficult to gain access to the corpus once the project is finished.In an ideal world, academics can turn to a sustainabilityinitiative in order to archive their datasets and to make themavailable to other researchers, e. g., by means of a web-basedcorpus platform (Dipper et al, 2006 & Schmidt et al 2006).Apart from issues such as providing standardised markuplanguages and metadata sets (Chiarcos et al, 2006 & Wörneret al, 2006), sustainability initiatives have to take the copyrightof the original data into account.We developed a tool that is able to mask corpora on the fly.Should someone who is interested in a corpus that is availableunder a rigid licence model not have a valid STC licence, heor she can still receive the corpus, albeit in masked form. Acorpus potentially can be associated with several accessibilityregulations: full access to TüBa-D/Z requires a licence for thetaz CD ROM, whereas masked versions can be placed under,say, the GNU Free Documentation or a Creative CommonsLicence. Therefore, a sustainability initiative has to come upwith a flexible system of representing the relationships anddependencies between the STC and the different annotationlayers and their individual licence restrictions.3 How to Mask Linguistic ResourcesThe easiest option to obfuscate an annotated corpus is to removethe text. A less radical solution substitutes every STC characterwith, for example, "x" and every digit with "0". In addition topreserving word length, this process retains information onupper and lower case by substituting these with "x" and "X"(Toms & Campbell, 1999).We developed CorpusMasker, a Java-based tool for theparameterised masking of linguistic resources represented asXML documents. The XML element(s) or attribute(s) thatcomprise the actual words or tokens to be masked (in case ofTüBa-D/Z, the <orth> element) can be specified to handlearbitrary annotation schemes. CorpusMasker features adictionary approach: after collecting all word forms, every wordis mapped onto a randomly generated string and replaced bythat string. Word length can be retained, as well as informationon the distribution and positioning of vowels and consonants.If a word is usually written with an initial lower case characterand that word appears with an initial upper case character, thesame randomised word is used (e. g., "dort" -> "kulp", "Dort"-> "Kulp"). CorpusMasker performs an affix analysis that issimilar tomorphology induction. The algorithm analyses certainwords, masks the roots, but retains the affixes, so that the textis masked but valuable linguistic information that in itself isinsufficient to reconstruct the source text or even to interpretthemasked text, is kept intact for further analysis. Parameterisedmasking can be performed with several different degrees ofretaining linguistic information, from the complete removal ofthe STC to a rather light but sufficient masking that keeps, e.g., closed word classes unchanged (see table 1; affixes aremarked in italics).3Linguistic corpora often contain POS information so that therandomisation process results in a list that could act as a keyto unlock the masked corpus, i. e., to reconstruct the STC. Aspublication of this complete list would contradict the purposeof the tool, we will only provide a reduced version of the fileso that the randomly generated words can bemapped onto POStags.Page 1674 Masked Corpora: What are They Good for?Our original goal had been to give researchers interested inTüBa-D/Z the option of examining the annotation withoutordering the die tageszeitung CD ROM first. As oursustainability platformwill give access to copyrighted corpora,we will implement the option of masking a corpus archivebefore every single download to enhance security. Furthermore,a password protected dictionary lookup could be provided thatenables researchers to retrieve a small amount of translationsfrom randomised strings back to original words. Following, wesketch some application scenarios for masked corpora.Unlexicalised parsing A masked corpus can be used for allsorts of unlexicalised training. Charniak (1996) shows that anunlexicalised PCFG trained on treebank annotations iscompatible with other unlexicalised parsers. In addition to themasked training data, a minimal amount of testing data wasrequired. In the case of TüBa-D/Z this subcorpus could consistof randomly shuffled example sentences from the treebank withunmasked text and full annotation. Hinrichs et al. (2005) discussexperiments in memory-based learning of anaphora resolution.Their tool is trained on the annotation of TüBa-D/Z and doesnot take lexical information into account. The features refer tomorphological properties, parts-of-speech, syntactic boundariesand grammatical functions, all of which are available in theannotation. In this case even the test data could be generateddirectly from the masked resource since the annotation includesmarking of equivalence classes comprising pronouns and nounphrases. The gold standard for testing consists of theseequivalence classes only in which the words are representedby positional indices. The evaluation would then test whetherthe relevant indices are grouped together correctly. Acomparable tool trained on masked corpus data could as wellbe applied to `real' German texts.Qualitative and quantitative analyses TüBa-D/Z's annotationcan be used for qualitative and quantitative analyses, it includesboth syntactic categories as well as grammatical functions. Alinguist can, for example, examine which categories occur aspredicatives (element PRED). In addition to this qualitativeinvestigation, the corpus also allows a quantitative analysis:what percentage of predicatives is realised by a noun phrase,what percentage is realised by an adjectival phrase or by aprepositional phrase? To give a second example, coordinatestructures are marked with the label KONJ; even withoutknowledge of the word level the treebank annotation givessufficent information to examine parallelism effects with respectto the structure of the conjuncts: syntactic categories,grammatical functions, modifiers, and length, see, e. g., Levy(2004), and Steiner (2006).Teaching linguistics and computational linguistics Themaskedversion of TüBa-D/Z contains an unnatural language that actslike German syntaxwise, but the lexicon of this languagecontains, for the most part, random strings and associated POStags. This fact makes the masked treebank a valuable resourcein the context of teaching computational linguistics. If studentshave to work with a language that has a known syntax and arudimentary morphology but lexical entries that bear nomeaning whatsoever, they might be able to concentrate betteron the tasks of developing grammar rules or improving parsingefficiency (e. g., with regard to unlexicalised parsing). Thisapproach of blanking out semantics is compatible withChomsky's notion of language as processing a set of symbols.4Evaluating NLP software Another promising applicationscenario is the evaluation of NLP software. Most tools usen-gram language models, more sophisticated applications canbe trained on annotated corpora. With a masked resource it ispossible to measure the influence syntactic annotations haveconcerning precision and recall, as the performance data of anNLP tool with regard to original, as well as slightly and fullymasked corpora can be compared. This approach could resultin substantial arguments in favour, or against the use oftreebanks for training NLP tools.5 Related WorkAnonymisation methods remove proper nouns and otheridentity-revealing phrases to protect the privacy of the peoplementioned in a text (for example, medical or legal records (Cortiet al, 2006,Medlock, 2006, Poesio et al, 2006, & Rock, 2001)).A second application area is concerned with the removal ofcues that might reveal the identity of the author of a text. Athird area concerns the masking, or obfuscation of texts, asdescribed in the present paper; we are not aware of similarapproaches to the masking of linguistic resources.56 Concluding RemarksWe call our approach parameterised masking because therandomisation process can be influenced with regard to severalparameters, so that, for example, certain word classes are notrandomised. Typically, when closed word classes such asdeterminers and prepositions are kept intact, at least part of theoriginal meaning of a sentence can be guessed. This leads usto a crucial question: what happens if we choose to mask onlya small number of words (for example, only proper nouns)?Page 168Do we have to mask a certain percentage of words, in order tobypass the STC's licensing restrictions? When does a text thathas beenmasked onlyminimally become the original text again,so that the licence restrictions prohibited the distribution of thepseudo-masked linguistic resource?AcknowledgementsThe authors would like to thank Timm Lehmberg (Hamburg)and Felix Zimmermann (Hannover) for valuable commentswith regard to legal aspects of the approach presented in thiscontribution. Furthermore, wewould like to thank our colleagueHolger Wunsch for valuable discussions.BibliographyCharniak, E. "Tree-bank Grammars." Proceedings of the 13thNational Conference on Artificial Intelligence (AAAI-96).MITPress, 1996. 1031-1036.Chiarcos, Christian, Timm Lehmberg, Georg Rehm, andAndreasWitt. Regulating Access to the Sustainability Platform.Technical report. SFB 441 (Tübingen University), 2006.Corti, L., A. Day, and G. Backhouse. "Confidentiality andInformed Consent: Issues for Consideration in the Preservationof and Provision of Access to Qualitative Data Archives."Forum: Qualitative Social Research 1.3 (2000).Dipper, S., E. Hinrichs, T. Schmidt, A. Wagner, and A. Witt."Sustainability of Linguistic Resources." Proceedings of theLREC 2006 Workshop Merging and Layering LinguisticInformation, Genoa, Italy. Ed. E. Hinrichs, N. Ide, M. Palmerand J. Pustejovsky. 2006. 48-54.Hinrichs, E., K. Filippova, and H. Wunsch. "What TreebanksCanDo For You: Rule-based andMachine-learningApproachesto Anaphora Resolution in German." Proceedings of the 4thWorkshop on Treebanks and Linguistic Theories (TLT 2005),Barcelona, Spain. Ed. M. Civit, S. Kübler and Ma. AntòniaMartí. 2005. 77-88.Levy, Roger. Presented at the Department of Linguistics,University of Colorado-Boulder, March 11, 2004. 2004.Medlock, B. "An Introduction to NLP-based TextualAnonymisation." Proceedings of the 5th InternationalConference on Language Resources and Evaluation (LREC2006), Genoa, Italy. Ed. N. Calzolari, K. Choukri, A. Gangemi,J. Mariani, B. Maegaard, J. Odjik and D. Tapias. 2006.1051-1056.Piez, Wendell. "Way Beyond Powerpoint: XML-driven SVGfor Presentations." Proceedings of XML 2004, Washington,November 2004. IDEA. Ed. Lauren Wood. 2004.Poesio, M., M. A. Kabadjov, P. Goux, U. Kruschwitz, E.Bishop, and L. Corti. "An Anaphora Resolution-BasedAnonymizationModule." Proceedings of the 5th InternationalConference on Language Resources and Evaluation (LREC2006), Genoa, Italy. Ed. N. Calzolari, K. Choukri, A. Gangemi,J. Mariani, B. Maegaard, J. Odjik and D. Tapias. 2006.1191-1193.Rock, F. "Policy and Practice in the Anonymisation ofLinguistic Data." International Journal of Corpus Linguistics6.1 (2001): 1-26.Schmidt, Thomas, Christian Chiarcos, TimmLehmberg, GeorgRehm, Andreas Witt, and Erhard Hinrichs. "Avoiding DataGraveyards: From Heterogeneous Data Collected in MultipleResearch Projects to Sustainable Linguistic Resources."Proceedings of the E-MELD 2006 Workshop on DigitalLanguage Documentation: Tools and Standards – The State ofthe Art, East Lansing, Michigan. 2006.Steiner, Ilona. "Coordinate Structures: On the Relationshipbetween Parsing Preferences and Corpus Frequencies."Pre-Proceedings of the International Conference on LinguisticEvidence 2006, Tübingen, February 2006. 2006.Telljohann, Heike, Erhard Hinrichs, Sandra Kübler, and HeikeZinsmeister. Stylebook for the Tübingen Treebank of WrittenGerman (TüBa-D/Z). Technical report. Tübingen: Seminar fürSprachwissenschaft, Universität Tübingen, 2006.Telljohann, Heinke, Erhard Hinrichs, and Sandra Kübler. " TheTüBa-D/Z Treebank – Annotating Germanwith a Context-FreeBackbone." Proceedings of the 4th International Conferenceon Language Resources and Evaluation (LREC 2004), Lisbon,Portugal, 2004. 2004.Toms, E. G., and D. G. Campbell. " Genre as InterfaceMetaphor: Exploiting Form and Function in DigitalEnvironments." Proceedings of the 32nd Hawaii InternationalConference on Systems Sciences (HICSS-32). IEEE ComputerSociety, 1999.Varga, D., P. Halácsy, A. Kornai, V. Nagy,, L. Németh, andV. Trón. "Parallel Corpora for Medium Density Languages."International Conference on Recent Advances in NaturalLanguage Processing, Borovets, Bulgaria, 2005. Ed. G.Angelova, K. Bontcheva, R. Mitkov, N. Nicolov and N.Nikolov. 2005. 590-596.Wörner, Kai, Andreas Witt, Georg Rehm, and Stefanie Dipper."Modelling Linguistic Data Structures."Proceedings of ExtremeMarkup Languages 2006,Montréal, Québec, August 2006.Ed.B. T. Usdin. 2006.1. The tazCDROM costs about 50 Euros. Licences for other corporaare often more expensive.Page 1692. The institution that created the annotation holds its copyright andcan decide the distribution conditions. As modern corpora maycomprise several annotation layers created by more than oneresearch group, each group can be considered the creator of itsannotation layer and can decide its terms of distribution (as aconsequence, every annotation layer should potentially comprisea complete metadata record). Commercially available softwaretools that were used in the annotation process might restrict theterms of distribution of the resulting data set as well.3. After DH 2007, a downloadable version of CorpusMasker will beavailable on our web site under an Open Source licence (<http://www.sfb441.uni-tuebingen.de/c2/>).4. For centuries, typographers and graphic designers use the "Loremipsum dolor sit amet" text fragment to evaluate new layouts withoutresorting to writing actual text. The blind text gives the impressionof a natural distribution of characters and whitespace withoutdistracting the reader by conveying any meaning that could beinterpreted intuitively. This approach might be useful forvisualising masked corpora by means of XML to SVGtransformations (Piez, 2004).5. In a message posted to Corpora-List on Aug 19th, 2006, PéterHalácsy suggested an interestingmethod to distribute a copyrightedcorpus under "fair use" conditions. Part of the copyright noticeHalácsy et al. apply to the Creative Commons-based licence ofthe "Hunglish" corpus (D. Varga et al, 2005) reads: "We preventedthe illegal use of copyrighted material by shuffling the texts atsentence level. This form is still useful for research purposes, whileit does not infringe upon the rightholders' interests."Page 170

Corpus Masking: Legally Bypassing Licensing Restrictions for the Free Distribution of Text Collections

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Corpus Masking: Legally Bypassing Licensing Restrictions for the Free Distribution of Text Collections

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