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Note of Exploiting Syntactic and semantic Information

  • August 2, 2010 8:57 pm

Paper: Exploiting Syntactic and Semantic Information for Relation Extraction from Wikipedia

This paper has two version, and the note here is take from the first version

The Semantic Web is based on RDF[3], a representation language using Notation 3 or N3[4]. We follow the formalism of Semantic Web, specifically N3, in which we structure Wikipedia’s content as a collection of statements. Each statement consists of a subject, a predicate and an object. … The statements with the use of a domain-specific ontology can then be straightforwardly transformed into RDF format that in turn serves as machine-processable knowledge base.

Our method, unlike other works, mines the key patterns from syntactic and semantic structure to measure similarity between entity pairs rather using only lexical information as in [5-8] or hard matching of dependency paths as in [9].

In details, we attempt to integrate syntactic and semantic information of text to form an unified structure. We then decompose the structure into subsequences and mine the frequent ones with the aim to capture the key patterns for each relationship.

2. Problem Statement

We aim at extracting binary relations between entities from English version of Wikipedia articles. A 2-tuple (ep, es) and a triple (ep,rel, es) denote an entity pair and a binary relation respectively,
where ep and es are entities which may be PERSON, ORGANIZATION, LOCATION, TIME OR ARTIFACT and
rel denotes the directed relationship between ep and es, which may be one of following 13 relations: CEO, FOUNDER, CHAIRMAN, COO, PRESIDENT, DIRECTOR, VICE CHAIRMAN, SPOUSE, BIRTH DATA, BIRTH PLACE, FOUNDATION, PRODUCT and LOCATION.

We follow [5] to define the entity mainly discussed in an article as principal entity, and other mentioned entities in the same article as secondary entities. We assume that interested entities in this problem should have a descriptive article in Wikipedia. Thus, no entity disambiguation and entity recognition is required in our system. The identifier of an entity is defined as the URL address to its appropriate article.

Our system predicts only the relations between the principal entity and each mentionded secondary entity in an article. As one more assumption, the relationship between an entity pair can be completely expressed in one sentence.

So that ,for an article, only the sentences that contain an principal entity and a secondary entity are necessarily to be analyzed.

4. Extract Relations from Wikipedia

4.1 Relation Extraction Framework

  1. articles should be processed to remove the HTML tags, extract hyperlinks which point to other wikipedia’s articles.passed to pre-processor: Sentence Splitter, Tokenizer and Phrase Chunker
  2. Parallelly processed to anchor all occurrences of principal entities and secondary entities. The Secondary Entity detector simply labels appropriate surface text of the hyperlinks as secondary entities.
  3. Sentence Selector chooses only sentences which contain the principal entity and at least one secondary entity. Each of such pairs becomes a relation candidate.
  4. The trainer receives articles with HTML tags to identify summary sections and extract ground truce relations annotated by human editors.
  5. Previously selected sentences that contain entity pairs from ground true relations are identified as training data.
  6. The trainer will learn the key patterns with respect to each relation.
  7. During testing, for each sentence and an entity pair on it, the Relation Extractor will identify the descriptive label and then outputs the final results.

Principal Entity Detector

  • Most of the pronouns in an article refer to the principal entity.
  • The first sentence of the article is often used to briefly define the principal entity.

We use rules to identify a set of referents to the principal entity, including three types[10]:

  • pronoun (“he”,”him”,”they”…)
  • proper noun (e.g., Bill Gates, William Henry Gates, Microsoft, …)
  • common nouns ( the company, the software, …)

Supported by the nature of Wikipedia, our technique performs better than those of the co-reference tools in LingPipe library and in OpenNLP tool set. All the occurrences of the collected referents are labeled as principal entity.

Training Data Builder

examine whether the pair is in ground true relation set or not. if yes, it attaches the relation label to the pair and create a new training sentence for the relation. For a relation r, the purpose of building training data is to collect the sentences that exactly express r. To reduce noise in training data, it is necessary to eliminate the pairs from the ground truce set which hold more than one relation.

4.2 Learning Patterns with Dependency Path

In this section, we will explain our first method to extracting relation using syntactic information.

Follow the idea in [9] we assume that the shortest dependency path tracing from a principal entity through the dependency tree to a secondary entity gives a concrete syntactic structure expressing relation between the pair.

Key patterns learning from the dependency paths for each relationship.

  1. derive dependency trees of the training sentences by Minipar parser and extract paths between entity pairs
  2. transform the paths into sequences which are in turn decomposed into subsequences
  3. From the subsequence collections of a relation r, we can identify the frequent subsequences for r.
  4. During testing, dependency path between an entity pair in an novel sentence is also converted into sequence and match with the previously mined subsequences.

Sequential Representation of Dependency Path

A word together with its Part-Of-Speech tag will be an element of the sequence.

Example

Learning Key Patterns as Mining Frequent Sequence

PrefixSpan, which is introduced in [12], is known as an efficient method to mining sequential patterns. A sequence s=s1s2…sn;, where si is an itemset, is called subsequence of a sequence p =p1p2…pm; if there exists integers 1 <= j12<…n<=m such that s1 ⊆ pj1, …, sn ⊆ pjn.

In this research, we use the implementation tool of PrefixSpan developed by Taku kudo.

From here, sequence database denotes the set of sequences converted from dependency paths with respect to a relation.

Weighting The Patterns

It is necessary for each mined pattern to be assigned a weight with respect to a relation for estimating the relevance. Factors:

  • Length of the pattern: if two paths share a long common subpattern, it is more likely that the paths express the same relationship.
  • Support of the pattern: is the number of sequences that contain the pattern. It is more likely that a pattern with high support should be a key pattern
  • Amount of lexical information: although the sequences contain both words and dependency relations from the original dependency path, we found that wordbased items are more important.
  • Number of sequence databases in which the pattern appear: if the pattern can be found in various sequence databases, it is more likely that the pattern is common and it should not be a key pattern of any relation.

wr(p) = ir f(p) x supportDr(p) x l(p) x elex(p)/|Dr|

Relation Selection

Given a novel sentence and the anchors of an entity pair in it, we will predict the appropriate relation of the pair. We extract the dependency path P, transform P into sequential pattern and then accumulate the scores of its subsequences for each relation r:

L_r(P)= \displaystyle\sum_{p in S(p)} \omega_{r (P)}

  • Lr(P) likelihood score to say that P expresses relation r
  • S(P) set of all subsequences of the sequential representation of P
  • The appropriate relation should be the one giving highest score to P:

R= \arg\displaystyle\max_r L_r(P)

4.3 Learning Patterns with Dependency Path and Semantic Role

We use the SNoW-based Semantic role Labeler [16], a state-of-art in SRL task which conforms the defintion of PropBank and CoNLL-2005 shared task on SRL. Since the SRL task just labels roles to constituents or phrases without indicateing which primitive concept playing the role, we still use dependency parsing information to further analyze the phrases.

[5] Integrating Probabilistic Extraction Models and Data Mining to Discover Relations and patterns in Text.

[9]Extracting Relations from Text

[10]Correference for nlp applications.

[12] Mining Sequential patterns by Pattern-Growth: The prefixspan approach.

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