GerNED: A German Corpus for Named Entity Disambiguation

Danuta Ploch, Leonhard Hennig, Angelina Duka, Ernesto William De Luca, Sahin Albayrak

DAI-Labor, Technische Universitat BerlinBerlin, Germany

{danuta.ploch,angelina.duka,sahin.albayrak}@dai-labor.de{leonhardhennig,ernesto.deluca}@googlemail.com

AbstractDetermining the real-world referents for name mentions of persons, organizations and other named entities in texts has become animportant task in many information retrieval scenarios and is referred to as Named Entity Disambiguation (NED). While comprehensivedatasets support the development and evaluation of NED approaches for English, there are no public datasets to assess NED systems forother languages, such as German. This paper describes the construction of an NED dataset based on a large corpus of German newsarticles. The dataset is closely modeled on the datasets used for the Knowledge Base Population tasks of the Text Analysis Conference,and contains gold standard annotations for the NED tasks of Entity Linking, NIL Detection and NIL Clustering. We also present firstexperimental results on the new dataset for each of these tasks in order to establish a baseline for future research efforts.

Keywords: German-Language Corpus, Named Entity Disambiguation, Cross-Document Coreference Resolution

1. IntroductionExtracting information from unstructured texts is an im-portant step towards the automatic creation of meaning-ful content representations, and is crucial in many areaslike information retrieval, topic detection and tracking, andknowledge base population. Named Entity Disambiguation(NED) is such an information extraction task, where thegoal is to determine the real-world referents of name men-tions in text (Bunescu and Pasca, 2006). It is related to wordsense disambiguation (Navigli, 2009) and cross-documentco-reference resolution (Bagga and Baldwin, 1998), but fo-cuses on the disambiguation of named entities such as per-sons, organizations, and geopolitical entities.NED systems typically address two main tasks, EntityLinking (Cucerzan, 2007) and NIL Clustering (Artiles etal., 2010). Entity Linking requires to accurately associatename mentions found in text to predefined entries of a ref-erence knowledge base (KB), and to recognize mentionsreferring to entities not covered by the KB (NIL detec-tion) (Dredze et al., 2010). As a step towards populating areference KB with new entries, the goal of the NIL Cluster-ing task is to group together name mentions of NIL queriesreferring to the same entity.NED is challenging since name mentions may be ambigu-ous, and entities can be referenced by different name vari-ants. For example, the name ‘Michael Jordan’ can refer tothe basketball player, but also to a researcher, as well asto many other people not covered by the reference KB. Onthe other hand, the basketball player Michael Jordan can bedesignated by his nickname ‘Air Jordan’ or simply by hisfamily name ‘Jordan’.The development and evaluation of NED approaches re-quire suitable corpora addressing these challenges and cov-ering a wide range of entities of different entity types. Fur-thermore, similar to most tasks that deal with processingnatural language text, it is desirable to develop and eval-uate NED methods that work well across different lan-guages, and that account for language-specific differences

and cross-lingual similarities. As the effort of constructingsuch resources is substantial, there are currently only veryfew larger NED corpora: the resources used in the Knowl-edge Base Population (KBP) track of the Text AnalysisConference (TAC) (Simpson et al., 2010), and the corporacreated for the Web People Search (WePS) challenges (Ar-tiles et al., 2010). However, both challenges focus on En-glish source material, and to the best of our knowledge,there is no comparable corpus for German-language NED.

Our Contributions: In this paper, we introduceGerNED, a German dataset for NED that consists of morethan 2,400 confusable name mentions found in a largecorpus of German news articles, and uses a referenceKB derived from the German version of Wikipedia.We describe the annotation procedure in Section 2, andoutline the characteristics of the corpus in terms of entitydistribution and confusability in Section 3. In Section 4 wedescribe an approach that uses standard NED algorithmsfor the tasks of Entity Linking and NIL Clustering. Finally,we present first experimental results on the new corpusfor each of the NED tasks in order to establish a baselinefor future research efforts in Section 5. The corpus will beavailable to the community from the authors upon request.

2. Resource CreationIn this section we describe the structure and the desiredqualities of the corpus, as well as the resources used. Then,we discuss our approach to selecting entities and creatingqueries.

2.1. Structure of the Corpus

Following the structure of the TAC-KBP evaluationdatasets, we created a German dataset for NED that con-sists of evaluation queries, gold-standard answers, a refer-ence knowledge base, and a source document corpus (seeFigure 1).

3886

Evaluation queries are specified by

• a query id,

• a surface form (name mention of an entity) and

• a source document id.

The surface form corresponds to a text string in the sourcedocument referring to a person (PER), organization (ORG),geopolitical entity (GPE) or an entity of an unknown type(UKN). The source document id is the file name of the newsarticle containing the surface form. For Entity Linking, asystem has to link each query to the correct knowledge baseentry, or decide that the query does not have a correspond-ing entry in the KB. For NIL Clustering, a system mustprovide a distinct NIL id for each set of co-referent NILqueries. We created gold-standard answers that map eachquery either to a unique entity id from the reference KB,or, in the case of NIL queries, to a distinct NIL id for eachunique entity. Example queries and gold-standard answersare shown in Figure 1.

2.2. GoalsThere are several desirable qualities for NED datasets, asoutlined by Simpson et al. (2010), which we consideredduring the construction of our corpus. Selected entitiesshould be confusable, i.e. they should share a name withanother another entity (ambiguity) or be referred to by a setof different name variants, such as spellings, nick namesor acronyms (synonymy). Ji et al. (2011) then define theoverall confusability of an NED dataset as:

ambiguity =

#surface forms referring to more than one entity

#surface forms

variety =

#entities expressed by more than one surface form

#entities

Furthermore, the dataset should cover different entity types(PER, ORG, GPE, and UKN), and contain entities withvarying mention frequency to cover popular and unpopularentities in the source document corpus. The source docu-ment corpus should contain sufficient occurrences of theseentities and their name variants. Another requirement isto have a sufficient number of NIL queries exhibiting simi-lar characteristics of name variance, ambiguity and mentionfrequency as KB queries. In order to allow for comparativeevaluations with existing corpora, we modeled the entitydistribution and confusability of our dataset on the EnglishTAC-KBP datasets (Simpson et al., 2010; Ji and Grishman,2011).

2.3. Source DataWe created the source document collection with thefriendly support of Neofonie GmbH by crawling webdocuments from more than 500 German news sourcesover a time period of seven months from 07/01/2010 to01/31/2011. News sources include national newspapers

(approx. 927,000 entries)

(881,501 German news articles from 07/01/2010 to 01/31/2011)

(approx. 2,500 queries)

… "Ziel dieses Prozesses ist eine

Schmerzensgeldzahlung wegen

übler Nachrede und Verleumd-

ung" , sagte Langer, "von uns

wurde ein Mindestbetrag von

150.000 Euro angesetzt.“ …

… "Ich habe gut gespielt und

einige wichtige Putts versenkt",

sagte Langer, dem die lange

Reise von Schottland nach

Sammamish im US-Bundesstaat

Washington offenbar nichts…

<query id="EL00720">

<name>Langer</name>

<docid>2010-07-31_4095946</docid>

</query>

<query id="EL00721">

<name>Langer</name>

<docid>2010-07-24_5738048</docid>

</query>

List of Queries

Query-ID Entity type Correct Answer

EL00720 PER http://de.wikipedia.org/wiki/Bernhard_Langer_(Golfer)

EL00721 PER NIL0071

Documents

(date: 05/31/2010)

Gold Standard Answers

Reference Knowledge Base

Figure 1: GerNED dataset components. The dataset pro-vides queries of name mentions in German news docu-ments. The gold standard answers link queries to the cor-rect entity id of the reference knowledge base or to a uniqueNIL id.

and magazines, local newspapers, and news agency feeds.They cover a wide range of news categories and genres,such as politics, financial news, and sports bulletins, andrange in length from multi-page essays to brief stock mar-ket bulletins. The time period for crawling the newswiredocuments was chosen to be close to, but later than the05/2010 epoch of the knowledge base to increase the like-lihood of being able to annotate entities not yet covered bythe Wikipedia-derived KB.The raw web documents were transformed into a struc-tured XML format with predefined elements such as title,teaser, and article text. Unwanted elements of the originalweb document, such as advertisements, navigation menus,footers, etc. were discarded. In total, the corpus contains881,501 news articles. We assume that a corpus of this sizecontains enough density and variety to cover a sufficientnumber of different entities and entity types, contexts ofvarying difficulty, as well as popular and unpopular enti-ties (Simpson et al., 2010).

2.4. Knowledge BaseThe reference knowledge base was constructed from theGerman Wikipedia. As discussed by Simpson et al. (2010),using Wikipedia has the advantage that Wikipedia entriescover many newsworthy entities. This facilitates choosingcandidate evaluation entities as they will very likely be rep-resented in a large corpus composed mainly of newswirearticles. We parsed a snapshot of the German Wikipediafrom 05/31/2010, and removed all disambiguation, redirectand other meta pages. The resulting reference KB containsapproximately 927K entries, as it includes not only namedentities, but also general encyclopedia entries.1 In this re-spect, our KB differs from the one used in the TAC-KBPevaluations, which consists only of Wikipedia pages havinginfoboxes. We did not generate separate entity identifiersfor each KB entry, but instead re-use the (unambiguous)Wikipedia URI of an entry. In addition, we stored the nor-

1Non-entity entries of the KB are ignored for the purpose ofentity selection during query creation.

3887

malized page title, as well as raw and cleaned-up versionsof the article’s text.2

2.5. Entity SelectionTo model the corpus closely on the TAC datasets, we usedqueries from the TAC-KBP 2010 training dataset as seeds.3

First, we translated the English surface forms to German us-ing Wikipedia interlanguage links, since the interlanguagelinks seem to be a reliable source for translations. Whereno interlanguage link was available, we kept the Englishsurface form or we translated name parts that are com-mon nouns by using a dictionary. Out of the different en-tity types, we had to translate geopolitical entities (GPE)most frequently, e.g. ‘Australia’ to ‘Australien’, ‘Bavaria’to ‘Bayern’ and so on, since GPEs often have language-dependent proper names. Usually, it wasn’t necessary totranslate proper names of persons and organizations sincethe names remain the same in both language. In some casesthough a translation to German provided additional surfaceforms. For example, both the surface form ‘Harvard Uni-versity’ and ‘Harvard Universitat’ are used in German newsarticles and therefore appropriate translations.After translating the surface forms we checked their avail-ability in the German KB and the source data. Surfaceforms found in news articles but missing in the KB servedas the basis for NIL queries. We decided to substituteor to complement less popular entities not occurring (of-ten enough) in our news corpus with equivalent Germanones. For example, we searched for a German comedianequivalent to an American one (Jerry Springer vs. HaraldSchmidt, DeGeneres vs. Engelke), a German town for anEnglish one (Lexington vs. Erfurt), etc.While creating new queries we ensured that Germanqueries had ambiguous surface forms and selected entitieswith several name variants like acronyms, abbreviations,and spelling mistakes, following the procedure of the TAC-KBP dataset construction process (Simpson et al., 2010).We included popular as well as less popular entities, andfocused on a high confusability. Besides queries for per-sons (PER), organizations (ORG), and geopolitical entities(GPE), we decided to create also queries for entities with anunknown type (UKN) in order to include interesting queriessuch as TV series sharing their name with persons or loca-tions.We found the annotation of NIL queries challenging. Espe-cially, the annotation of GPEs proved difficult, since in con-trast to persons and organizations almost every GPE foundin a news article was covered by the KB. Out-of-KB-GPEsare therefore under-represented in our corpus. To find NILqueries we followed two strategies: On the one hand weannotated less popular entities sharing the same name witha popular entity to augment the confusability of the queriesand to reduce the bias towards popular entities. On the otherhand we searched for novel entries added after 05/31/2010to the German Wikipedia, and included them as a query ifthey occurred in our news corpus.

2We keep the raw article text including markup to allow forlater parsing, for example to determine an entity’s type from itsinfobox, or to collect the contextual links of an article.

3http://nlp.cs.qc.cuny.edu/kbp/2010/

Figure 2: A screenshot of the annotation tool. The selecteditems form a query.

2.6. Query Creation

We provided an annotation tool (Figure 2) and instructedannotators to create queries by searching for a surface form,selecting a document containing it, and linking it to the cor-rect entity from a list of candidate entities. To create a con-fusable corpus in terms of ambiguity and variety the anno-tators were advised to augment the initial surface forms listand if possible to create numerous queries per surface formand entity. They were asked to select different entities persurface form and also to annotate different name variantsfor one entity using name variants found in Wikipedia or inthe news corpus. If a surface form referred to a NIL entity,annotators could create a novel entity or select from the setof previously created NIL entities. Annotators were alsoadvised to specify the entity type and to select only docu-ments containing at least one surface form exactly match-ing the query. This means, to create e.g. a query containingonly a person’s family name, the selected document mustcontain at least one mention of the person by just his orher family name. Altogether, three annotators created thequeries. Although each annotator created a different set ofqueries, all queries were checked later by at least one otherannotator. Inconclusive queries were removed from the cor-pus.

Figure 2 shows a screenshot of the provided annotationtool. The view on the left hand side displays surface forms,the center view lists all documents containing the selectedsurface form, and view on the right hand side potential can-didate entities, as determined by a KB lookup. If an entityis missing in the candidate list, the annotators have the pos-sibility to add the correct KB entry to the list. Workingon a NIL query, the tool offers an internal KB with NILentities so that the annotators either can select an existingNIL entity or add a new entity to the KB of NIL entities.We generate NIL entity identifiers by starting with the id‘NIL0001’ and then incrementing the identifiers by one foreach new NIL entity. The selected items, as displayed inFigure 2, constitute a single query of the corpus.

3888

All KB NILPER 700 450 250ORG 1127 615 512GPE 563 542 21UKN 78 57 21ANY 2468 1664 804

Table 1: Query distribution of the GerNED corpus

All KB NILGerNED corpus 2468 1664 804TAC-KBP 2010 train 1500 1074 426TAC-KBP 2010 eval 2250 1020 1230TAC-KBP 2011 eval 2250 1124 1126

Table 2: Distribution of KB and NIL queries in the GerNEDcorpus in comparison to the TAC-KBP datasets

3. Corpus StatisticsThis section summarizes the key characteristics of the cre-ated German NED corpus such as the size, the distributionof KB and NIL queries and of different entity types, andcompares them to the TAC-KBP datasets.The German corpus contains a total of 2468 queries, with1664 KB and 804 NIL queries. Table 1 shows the dis-tribution of entity types in the corpus for NIL, KB andall queries. The majority of queries (46%) relate to or-ganizations, 28% of the the queries relate to persons and23% to geopolitical entities. Only 3% are of an unknowntype. Altogether, the corpus provides queries for 1190unique named entities and 1098 distinct surface forms de-tected in 2417 news articles crawled from 27 German newsproviders.The distribution of 70% KB and 30% NIL queries and ofdifferent entity types in the GerNED corpus does not reflectnecessarily their distribution in news articles, but results ina corpus with an intended focus on confusable queries. Ta-bles 2 and 3 compare the entity distribution of the Germandataset with the TAC-KBP datasets of 2010 and 2011.

3.1. Surface FormsThe annotators succeeded in creating numerous queries ref-erencing different entities by the same surface form. Onaverage, each surface form refers to 1.21 entities. Approxi-mately 15% of the surface forms in the corpus are annotatedwith more than one entity. These queries cover different de-grees of difficulty ranging from ambiguous names for enti-ties of the same type (different people named ‘Schmidt’),for entities of different types (‘Duke Energy’ vs. ‘MikeDuke’) to surface forms used as metonyms – which occurquite often in the created dataset. For example, the sur-face form ‘Erfurt’ relates to the town in Thuringia, but mayalso be used to denote the town’s football club. The mostambiguous surface forms in the corpus are ‘Duke’, ‘Er-furt’, ‘UC’, ‘MGM’, ‘San Diego’, ‘Vancouver’, ‘Schmidt’,‘Weißensee’ and ‘Justizministerium’.

5Ambiguity and variety cannot be computed for the full TAC-KBP 2010 training dataset as its NIL queries are not annotatedwith distinct NIL identifiers.

PER ORG GPE UKNGerNED corpus 700 1127 563 78TAC-KBP 2010 train 500 500 500 -TAC-KBP 2010 eval 751 750 749 -TAC-KBP 2011 eval 750 750 750 -

Table 3: Distribution of entity types in queries of theGerNED corpus in comparison to the TAC-KBP datasets

All KB NILGerNED corpus 14.57 % 15.80 % 7.16 %TAC-KBP 2010 train N/A 4.12 % N/ATAC-KBP 2010 eval 12.93 % 5.70 % 9.31 %TAC-KBP 2011 eval 13.23 % 12.42 % 7.15 %

Table 4: Ambiguity of the German corpus in comparison tothe TAC-KBP datasets5

All KB NILGerNED corpus 11.09 % 8.39 % 14.90 %TAC-KBP 2010 train N/A 3.90 % N/ATAC-KBP 2010 eval 1.95 % 2.49 % 1.49 %TAC-KBP 2011 eval 1.12 % 1.56 % 0.74 %

Table 5: Variety of the German corpus in comparison to theTAC-KBP datasets5

Table 4 summarizes the ambiguity of the German corpusas well as of the TAC-KBP datasets for NIL, KB and Allqueries. Figure 3 shows a detailed overview of the surfaceform distribution. It illustrates the proportions of surfaceforms for which one, two, three or more than three namedentities are annotated and compares the ambiguity of thesurface forms with the TAC-KBP datasets. A surface formis covered on average by 2.3 queries and 64% of the surfaceforms are annotated in more than one query. Table 4 andFigure 3 show that the ambiguity of the GerNED corpus iscomparable to the ambiguity of the TAC-KBP datasets.

3.2. Named EntitiesThe corpus contains a number of entities denoted by dif-ferent name variants. On average, each entity is referredto by 1.12 distinct surface forms. Table 5 shows the vari-ety of the German corpus in comparison to the TAC-KBPdatasets. It considers the variety of the entire query set aswell as of NIL and KB queries and points out that especiallythe variety of NIL queries is very high. Overall, the anno-tated queries cover various name variants like acronyms,spelling mistakes and multilingual names. For example,the NATO organization is annotated with the English sur-face form ‘North Atlantic Treaty Organization’ and with theGerman surface form ‘Organisation des Nordatlantikver-trags’, both occurring in German news articles. Regard-ing spelling mistakes, the GerNED corpus provides queriessuch as ‘Rotterdamm’ for the city in the Netherlands. Someentities are also annotated with their acronyms. For exam-ple, the ‘Weltgesundheitsorganisation’ can be referencedby the acronym ‘WHO’. Figure 4 shows that 9.8% of allentities in the German corpus are annotated with two dis-tinct surface forms and 1.3% of the entities are annotatedwith three surface forms. This usage of synonyms is con-

3889

85

.4

9.9

2.8

1.8

86

.8

11

.3

1.3

0.7

87

.1

9.3

2.3

1.3

95

.9

3.9

0.2

0.0

0

10

20

30

40

50

60

70

80

90

100

1 2 3 >3

Pe

rce

nta

ge o

f SF

Entities per SF

German NED dataset

TAC 2011 evaluation

TAC 2010 evaluation

TAC 2010 training

Figure 3: Entities per surface form in comparison to otherTAC-KBP datasets

88

.9

9.8

1.3

98

.9

1.1

0.0

98

.0

2.0

0.0

96

.1

3.7

0.2

0

10

20

30

40

50

60

70

80

90

100

1 2 3

Pe

rce

nta

ge o

f En

titi

es

SFs per Entity

German NED dataset

TAC 2011 evaluation

TAC 2010 evaluation

TAC 2010 training

Figure 4: Surface forms per entity in comparison to otherTAC-KBP datasets

siderably higher than in the TAC-KBP datasets. The enti-ties with the highest name variety are ‘Sido’, ‘Myanmar’,‘Phoenix Hagen’, ‘Welthandelsorganisation’ and ‘HarvardUniversity’. Furthermore, there are 2.1 queries per entityon average, and 60% of the entities are annotated in morethan one query.

4. Baseline Approach to NEDIn this section we describe an approach that uses standardNED algorithms in order to establish a baseline on the pre-sented dataset for future research efforts. We implement thesubtasks of Entity Linking and NIL Clustering as a two-stepprocess, first detecting queries referencing a KB entry, andthen clustering the remaining NIL queries. This approachfollows common practice and is implemented by many sys-tems participating in the TAC-KBP tracks (Ji and Grish-man, 2011).

4.1. Entity linkingWe formulate Entity Linking as a supervised classificationproblem. We first generate a set of candidate KB entries fora query, and then rank candidates according to the likeli-hood that they correspond to the correct entry. Finally, weemploy another classification step to detect queries refer-ring to NIL entities. In the following we will briefly de-scribe this approach, for more details we refer the reader toPloch (2011) and Ploch et al. (2011).We generate candidates by collecting name variants foreach KB entry from article titles, redirect pages, disam-biguation pages and the anchor texts of internal Wikipedialinks in a preprocessing step. We normalize name variantsby lower-casing, and removing punctuation as well as ap-positives. Candidate generation is then performed by look-ing up the query name mention in an inverted index map-ping name variants to KB entries. This step is geared to-wards high recall, and prefers a larger candidate set overa smaller one. We limit the candidate set to the N highestscoring results according to the relevance score computedby the index search.6

In order to rank candidates, we represent each candidateas a feature vector encoding contextual and KB knowledgeas well as comparisons of the two. To provide a realistic

6In our experiments, N = 100 was set based on evaluations onthe TAC-KBP datasets.

baseline, we implement three well-known features whichhave been shown to be very useful in Entity Linking. Thefirst feature, surface form popularity (SFP) is a KB featurethat encodes the likelihood with which a particular surfaceform refers to a given target entity. The entity distributionfor a given surface form is determined from the link fre-quencies of internal Wikipedia anchors, including redirectand disambiguation pages. This feature captures the prefer-ence for the “most frequent sense” of a name mention (Hanand Zhao, 2009). The second feature is based on our useof an inverted index for candidate generation. The candi-date selection score (CS) measures the relevance score ofeach KB entity as calculated by the weighted index search,which uses a modified tf-idf weighting scheme over the dif-ferent parts of a name mention. We found this feature to bevery useful in our experiments on KBP 2009 and KBP 2010datasets, see also Ploch (2011). Our last feature measuresthe bag-of-words (BOW) similarity between the query doc-ument and a candidate’s KB text using the cosine similar-ity of tf-idf-weighted word vector representations (Bunescuand Pasca, 2006). We preprocessed document and articletexts by performing stemming using Porter’s stemmer andremoving words occurring in a stop word list.The NIL detection classifier is based on features derivedfrom the atomic features of all candidates of a given query.We calculate several different features, such as the max,mean, min, max-mean, and max-min, of the atomic features,using the feature vectors of all candidates of a query.

4.2. NIL Clustering

Our approach for NIL Clustering is based on a hierarchicalagglomerative clustering (HAC) algorithm which is a com-mon approach to the task of clustering documents accord-ing to the entities they mention (Artiles et al., 2010). TheHAC algorithm first assigns each query to its own clusterand then successively merges pairs of clusters until a pre-defined similarity threshold t is reached or until all queriesare assigned to a single cluster. In our baseline scenariowe use single-link clustering. We measure the similaritybetween two queries by calculating the cosine similaritybetween the tf-idf-weighted word vectors constructed fromthe document texts of the queries. As for the task of EntityLinking, we first perform stemming and remove stop wordsbefore creating the word vectors.

3890

In addition to the HAC baseline approach, we apply threemore baseline algorithms to cluster NIL queries. The firsttwo baselines are the straightforward clustering approachesone-in-one and all-in-one that assign each query to its owncluster or all queries to one single cluster, respectively. An-other standard clustering approach for NIL Clustering is togroup the queries according to their name mentions. Tothis end, we lower-case all name mentions and cluster allqueries sharing the same name (all-for-sf ). We providethese baselines to examine whether the created corpus isrobust enough to avoid ‘cheating’.

5. EvaluationThis section presents experimental results of the baselineapproach on the GerNED corpus. We evaluate the stepsof Entity Linking and NIL Clustering separately, and addi-tionally conduct an evaluation run considering both steps,where the output of the Entity Linking task is passed as in-put for the NIL Clustering step.We measure the quality of this baseline approach using es-tablished performance measures adopted in the TAC-KBPEntity Linking task, namely the micro-averaged accuracy(MAA) and the Bˆ3+ metric (Ji and Grishman, 2011). MAAis query-oriented, and measures the fraction of correctlylinked queries, whereas Bˆ3+ evaluates the correctness ofthe clusters of queries referring to the same entity.7

5.1. Model Training and Parameter SelectionFor Entity Linking, we randomly split the 2468 queries ofour dataset into five folds to perform cross-validation. Eachsplit uses 60% of the data for training, 20% for validation,and the remaining 20% of the data for testing. We stratifythe folds to ensure a similar distribution of KB and NILqueries, and normalize feature values.We use a Support Vector Machine classification algo-rithm (Vapnik, 1995) to train models for candidate rank-ing and NIL detection, utilizing the LibSVM implemen-tation (Chang and Lin, 2001). For training the candidateranking classifier we label as a positive example at mostone candidate from the set of candidates for a given query,and all others as negative. For training the NIL classifier,we create a single feature vector per query, which we labelas positive if the query refers to a NIL entity. Both classi-fiers use a radial basis function kernel. In each iteration, weperform a grid search to determine optimal values for theSVM’s hyperparameters C and γ. The classifier modelswith optimal performance on the validation data are thenused for testing. Results reported in this paper are averagedacross the test folds.We evaluate the baseline NIL Clustering algorithms in twodifferent experimental setups. The first experiment mea-sures the performance when clustering gold-standard NILqueries, in order to avoid a skewed NIL Clustering score re-sulting from noise introduced by previous NED steps. Thesecond experiment uses the answers predicted by the base-line entity linker to assess the performance of the overallbaseline system. We cluster only queries classified as ‘NIL’

7Scorer available at: http://nlp.cs.qc.cuny.edu/kbp/2011/scoring.html

MAA Bˆ3+ Prec Bˆ3+ Rec Bˆ3+ F1KB 0.627 0.597 0.584 0.590NIL 0.910 0.765 0.755 0.758ALL 0.719 0.643 0.639 0.641

Table 6: Bˆ3+ scores and micro-averaged accuracy for theNED baseline system on the GerNED dataset.

MAA Bˆ3+ Prec Bˆ3+ Rec Bˆ3+ F1PER 0.744 0.731 0.663 0.695GPE 0.760 0.747 0.727 0.737ORG 0.712 0.615 0.610 0.612UKN 0.294 0.291 0.237 0.261

Table 7: Bˆ3+ scores and micro-averaged accuracy forthe NED baseline system by entity type, on the GerNEDdataset.

by the entity linker. Queries already linked to the KB are ig-nored during NIL Clustering but considered for calculatingthe overall evaluation score.To tune the threshold parameter t of the HAC algorithm,we randomly split the queries of the dataset into five folds.We ensure that queries for one entity are not distributedacross different folds and that the distribution of NIL andKB queries corresponds to their original distribution. Toevaluate the HAC approach, we perform cross-validationwhere we use 20% of the data for finding a good value ofthe parameter t, and the remaining 80% for testing. TheNIL Clustering results are then averaged across the testfolds. We evaluate all other NIL Clustering baseline algo-rithms on 100% of the queries, depending on the evaluationscenario on gold-standard NIL queries or on all queries.

5.2. ResultsWe present the results of our baseline Entity Linking systemin Tables 6 and 7. Table 6 shows the micro-averaged accu-racy (MAA) and Bˆ3+ scores for all queries, KB queriesonly, and NIL queries only. The baseline system achievesan MAA score of 0.719 and a Bˆ3+ F1 score of 0.641 whenconsidering all queries. MAA and Bˆ3+ scores for NILqueries are significantly higher than for KB queries. Thebetter performance on NIL queries suggests that the cho-sen features are good indicators for discriminating betweenentities known respectively unknown to the KB, but do notalways result in a correct ranking of candidate entities.In Table 7, we give detailed performance statistics of thebaseline NED approach for different entity types. From thetable, we see that MAA scores are quite similar for PER,GPE and ORG entities, with GPE and PER entities beingslightly easier than ORG entities. Entities of type UKN,however, are much harder to link, and queries for UKNentities only have an MAA score of 0.294. Again, Bˆ3+scores reasonably mirror MAA scores, with the lower per-formance for ORG entities (compared to PER, GPE) some-what more evident. The results shown in this table suggestthat the entities selected for the dataset are of a similar dif-ficulty across entity types, with the exception of the muchharder queries for UKN entities.Table 8 summarizes the Bˆ3+ F1 results of the four base-

3891

all-in-one one-in-one all-for-sf HACNIL 0.006 0.719 0.895 0.843All 0.501 0.615 0.654 0.641

Table 8: Bˆ3+ F1 scores for NIL Clustering baselines onthe GerNED dataset.

MAA Bˆ3+ Bˆ3+ Bˆ3+Prec Rec F1

GerNED 0.719 0.643 0.639 0.641TAC-KBP 2010 eval 0.776 0.638 0.583 0.609TAC-KBP 2011 eval 0.697 0.637 0.596 0.616

Table 9: Comparison of Bˆ3+ scores and micro-averagedaccuracy for the baseline system on different NED datasets.

line approaches to NIL Clustering. The first row listsresults obtained performing the clustering algorithms ongold-standard NIL queries. It shows that the all-in-one ap-proach with its Bˆ3+ F1 score of 0.006 is not suitable forthe dataset. The one-in-one approach achieves significantlybetter results. This can be expected due to the structure ofthe corpus which consists predominantly of small clusters.On average, a cluster contains 1.8 queries and 75% of theclusters consist of only one or two queries. The best resultsare achieved by the all-for-sf approach. It performs evenbetter than the HAC approach which is part of the base-line system. Still, the Bˆ3+ F1 score of 0.895 offers roomfor improvement. The second row of the table reports re-sults of the baseline system including Entity Linking andNIL Clustering. The results mirror the trend between theclustering approaches evaluated on NIL queries. Since themajority of the queries can be linked to the KB and the NILqueries are well separable by their surface form, the moreimportant subtask is therefore the Entity Linking step.In order to establish a context for the results of the NEDbaseline, Table 9 compares the system’s performance ondifferent datasets.8 Bˆ3+ scores are comparable across thethree datasets. MAA scores on the TAC-KBP 2010 evaldataset are higher than on the TAC-KBP 2011 eval dataset,and also higher than on the GerNED corpus. A similarobservation was made by Ji et al. (2011), who noted thatKBP2011 systems perform generally worse on 2011 datathan on 2010 data. Overall, the results shown in Table 9suggest that the GerNED corpus is of similar difficulty asrecent TAC-KBP datasets.

6. Related WorkThe most prominent resources for the task of Named En-tity Disambiguation are the datasets distributed for the TextAnalysis Conference’s Knowledge Base Population (TAC-KBP) track (Simpson et al., 2010). These datasets pro-vide training data for Entity Linking, NIL Detection andNIL Clustering for English queries and newswire text.The TAC 2011 evaluation in addition included annotateddata for cross-lingual Entity Linking using Chinese queriesand source documents together with an English knowledge

8The evaluation on the TAC-KBP datasets was conducted inthe same manner as on the GerNED dataset, i.e. using cross-validation and averaging results across folds.

base (Ji et al., 2011). Recently, Mayfield et al. (2011) pre-sented a dataset for cross-lingual Entity Linking that mapsEnglish name mentions to non-English documents. Thedataset contains approximately 55,000 queries for docu-ments in 21 different non-English languages.The task of clustering entities without reference to a knowl-edge base is also addressed by the WEPS challenges (Ar-tiles et al., 2010), which focus on grouping distinct PERentities referenced in web documents retrieved by queryinga search engine for person names.

7. ConclusionsWe presented a novel, German-language corpus for the taskof Named Entity Disambiguation. The corpus consists ofa large set of newswire documents, a Wikipedia-derivedknowledge base, and a set of queries for ambiguous namementions. It provides annotations for the subtasks of link-ing named entity mentions in documents to the knowledgebase, and of clustering name mentions of entities not foundin the knowledge base according to their real-world refer-ents. We plan to make this corpus available to the largerresearch community.Our analysis shows that our corpus is of similar confusabil-ity as the TAC-KBP datasets. It contains a higher fraction ofsynonymous queries than the TAC-KBP datasets, while be-ing comparable in terms of query ambiguity. The Germancorpus contains fewer NIL queries because of Wikipedia’sextensive coverage of named entities. In particular, novelgeopolitical entities were hard to find.Experiments using well-known baseline algorithms for En-tity Linking and NIL Clustering give a micro-averaged ac-curacy of 0.719, and a Bˆ3+ F1 score of 0.641 on the pre-sented dataset. These figures suggest that the GerNED cor-pus is of similar difficulty as the TAC-KBP datasets, andleave room for more sophisticated approaches.In future work we intend to expand the NED corpus andaugment the source data with non-news documents such asmicro-blogs or public sector information in order to studyNED in these contexts.

8. ReferencesJavier Artiles, Andrew Borthwick, Julio Gonzalo, Satoshi

Sekine, and Enrique Amigo. 2010. WePS-3 evaluationcampaign: Overview of the web people search clusteringand attribute extraction tasks. In Proc. of CLEF 2010.

Amit Bagga and Breck Baldwin. 1998. Entity-based cross-document coreferencing using the vector space model.In Proc. of COLING 1998, pages 79 – 85.

Razvan Bunescu and Marius Pasca. 2006. Using encyclo-pedic knowledge for named entity disambiguation. InProc. of EACL 2006, pages 9–16.

Chih-Chung Chang and Chih-Jen Lin, 2001. LIB-SVM: a library for support vector machines. Soft-ware available at http://www.csie.ntu.edu.tw/˜cjlin/libsvm.

Silviu Cucerzan. 2007. Large-Scale named entity disam-biguation based on Wikipedia data. In Proc. of EMNLP-CoNLL 2007, pages 708–716.

3892

Mark Dredze, Paul McNamee, Delip Rao, Adam Gerber,and Tim Finin. 2010. Entity disambiguation for knowl-edge base population. In Proc. of COLING 2010, pages277–285.

Xianpei Han and Jun Zhao. 2009. Named entity disam-biguation by leveraging wikipedia semantic knowledge.In Proc. of CIKM 2009, pages 215–224.

Heng Ji and Ralph Grishman. 2011. Knowledge base pop-ulation: Successful approaches and challenges. In Proc.of ACL-HLT 2011, pages 1148–1158.

Heng Ji, Ralph Grishman, and Hoa Trang Dang. 2011.Overview of the TAC2011 knowledge base populationtrack. In Proc. of TAC 2011.

James Mayfield, Dawn Lawrie, Paul McNamee, and Dou-glas Oard. 2011. Building a Cross-Language entity link-ing collection in Twenty-One languages. In Proc. ofCLEF 2011, pages 3–13.

Roberto Navigli. 2009. Word sense disambiguation: A sur-vey. ACM Comput. Surv., 41(2):1–69.

Danuta Ploch, Leonhard Hennig, Ernesto William De Luca,and Sahin Albayrak. 2011. DAI approaches to the TAC-KBP 2011 entity linking task. In Proc. of TAC 2011.

Danuta Ploch. 2011. Exploring entity relations for namedentity disambiguation. In Proc. of ACL 2011 (StudentSession), pages 18–23.

Heather Simpson, Stephanie Strassel, Robert Parker, andPaul McNamee. 2010. Wikipedia and the web of con-fusable entities: Experience from entity linking querycreation for TAC 2009 knowledge base population. InProc. of LREC’10.

Vladimir N. Vapnik. 1995. The nature of statistical learn-ing theory. Springer, New York.

3893