Knowledge extraction

Knowledge extraction is the creation of knowledge from structured (relational databases, XML) and unstructured (text, documents, images) sources. The resulting knowledge needs to be in a machine-readable and machine-interpretable format and must represent knowledge in a manner that facilitates inferencing. Although it is methodically similar to information extraction (IE) in natural language processing (NLP) and extract, transform, load (ETL), the main criterion is that the extraction result goes beyond the creation of structured information or the transformation into a relational schema. It requires either the reuse of existing formal knowledge (reusing identifiers or ontologies) or the generation of a schema based on the source data. The RDB2RDF W3C group was standardizing a language for extraction of resource description frameworks (RDF) from relational databases. Another popular example for knowledge extraction is the transformation of Wikipedia into structured data and also the mapping to existing knowledge (see DBpedia and Freebase).

Knowledge extraction is the creation of knowledge from structured (relational databases, XML) and unstructured (text, documents, images) sources. The resulting knowledge needs to be in a machine-readable and machine-interpretable format and must represent knowledge in a manner that facilitates inferencing. Although it is methodically similar to information extraction (IE) in natural language processing (NLP) and extract, transform, load (ETL), the main criterion is that the extraction result goes beyond the creation of structured information or the transformation into a relational schema. It requires either the reuse of existing formal knowledge (reusing identifiers or ontologies) or the generation of a schema based on the source data. The RDB2RDF W3C group was standardizing a language for extraction of resource description frameworks (RDF) from relational databases. Another popular example for knowledge extraction is the transformation of Wikipedia into structured data and also the mapping to existing knowledge (see DBpedia and Freebase). ==Overview== After the standardization of knowledge representation languages such as RDF and OWL, much research has been conducted in the area, especially regarding transforming relational databases into RDF, identity resolution, knowledge discovery and ontology learning. The general process uses traditional methods from information extraction and extract, transform, and load (ETL), which transform the data from the sources into structured formats. So understanding how the interact and learn from each other. The following criteria can be used to categorize approaches in this topic (some of them only account for extraction from relational databases): ==Examples== ===Entity linking=== DBpedia Spotlight, OpenCalais, Dandelion dataTXT , the Zemanta API, Extractiv and PoolParty Extractor analyze free text via named-entity recognition and then disambiguates candidates via name resolution and links the found entities to the DBpedia knowledge repository (Dandelion dataTXT demo or DBpedia Spotlight web demo or PoolParty Extractor Demo). President Obama called Wednesday on Congress to extend a tax break for students included in last year's economic stimulus package, arguing that the policy provides more generous assistance. As President Obama is linked to a DBpedia LinkedData resource, further information can be retrieved automatically and a Semantic Reasoner can for example infer that the mentioned entity is of the type Person (using FOAF (software)) and of type Presidents of the United States (using YAGO). Counter examples: Methods that only recognize entities or link to Wikipedia articles and other targets that do not provide further retrieval of structured data and formal knowledge. ===Relational databases to RDF=== Triplify, D2R Server, Ultrawrap , and Virtuoso RDF Views are tools that transform relational databases to RDF. During this process they allow reusing existing vocabularies and ontologies during the conversion process. When transforming a typical relational table named users, one column (e.g.name) or an aggregation of columns (e.g.first_name and last_name) has to provide the URI of the created entity. Normally the primary key is used. Every other column can be extracted as a relation with this entity. Then properties with formally defined semantics are used (and reused) to interpret the information. For example, a column in a user table called marriedTo can be defined as symmetrical relation and a column homepage can be converted to a property from the FOAF Vocabulary called foaf:homepage, thus qualifying it as an inverse functional property. Then each entry of the user table can be made an instance of the class foaf:Person (Ontology Population). Additionally domain knowledge (in form of an ontology) could be created from the status_id, either by manually created rules (if status_id is 2, the entry belongs to class Teacher ) or by (semi)-automated methods (ontology learning). Here is an example transformation: Peter :marriedTo :Mary . marriedTo a owl:SymmetricProperty . Peter foaf:homepage . Peter a foaf:Person . Peter a :Student . Claus a :Teacher . ==Extraction from natural language sources== The largest portion of information contained in business documents (about 80% ) is encoded in natural language and therefore unstructured. Because unstructured data is rather a challenge for knowledge extraction, more sophisticated methods are required, which generally tend to supply worse results compared to structured data. The potential for a massive acquisition of extracted knowledge, however, should compensate the increased complexity and decreased quality of extraction. In the following, natural language sources are understood as sources of information, where the data is given in an unstructured fashion as plain text. If the given text is additionally embedded in a markup document (e. g. HTML document), the mentioned systems normally remove the markup elements automatically. ===Traditional information extraction (IE)=== Traditional information extraction is a technology of natural language processing, which extracts information from typically natural language texts and structures these in a suitable manner. The kinds of information to be identified must be specified in a model before beginning the process, which is why the whole process of traditional Information Extraction is domain dependent. The IE is split in the following five subtasks. Named entity recognition (NER) Coreference resolution (CO) Template element construction (TE) Template relation construction (TR) Template scenario production (ST) The task of named entity recognition is to recognize and to categorize all named entities contained in a text (assignment of a named entity to a predefined category). This works by application of grammar based methods or statistical models. Coreference resolution identifies equivalent entities, which were recognized by NER, within a text. There are two relevant kinds of equivalence relationship. The first one relates to the relationship between two different represented entities (e.g. IBM Europe and IBM) and the second one to the relationship between an entity and their anaphoric references (e.g. it and IBM). Both kinds can be recognized by coreference resolution. During template element construction the IE system identifies descriptive properties of entities, recognized by NER and CO. These properties correspond to ordinary qualities like red or big. Template relation construction identifies relations, which exist between the template elements. These relations can be of several kinds, such as works-for or located-in, with the restriction, that both domain and range correspond to entities. In the template scenario production events, which are described in the text, will be identified and structured with respect to the entities, recognized by NER and CO and relations, identified by TR. ===Ontology learning (OL)=== Ontology learning is the automatic or semi-automatic creation of ontologies, including extracting the corresponding domain's terms from natural language text. As building ontologies manually is extremely labor-intensive and time-consuming, there is great motivation to automate the process. ===Semantic annotation (SA)=== During semantic annotation, natural language text is augmented with metadata (often represented in RDFa), which should make the semantics of contained terms machine-understandable. At this process, which is generally semi-automatic, knowledge is extracted in the sense, that a link between lexical terms and for example concepts from ontologies is established. Thus, knowledge is gained, which meaning of a term in the processed context was intended and therefore the meaning of the text is grounded in machine-readable data with the ability to draw inferences. Semantic annotation is typically split into the following two subtasks. Terminology extraction Entity linking At the terminology extraction level, lexical terms from the text are extracted. For this purpose a tokenizer determines at first the word boundaries and solves abbreviations. Afterwards terms from the text, which correspond to a concept, are extracted with the help of a domain-specific lexicon to link these at entity linking. In entity linking a link between the extracted lexical terms from the source text and the concepts from an ontology or knowledge base such as DBpedia is established. For this, candidate-concepts are detected appropriately to the several meanings of a term with the help of a lexicon. Finally, the context of the terms is analyzed to determine the most appropriate disambiguation and to assign the term to the correct concept. Note that "semantic annotation" in the context of knowledge extraction is not to be confused with semantic parsing as understood in natural language processing (also referred to as "semantic annotation"): Semantic parsing aims a complete, machine-readable representation of natural language, whereas semantic annotation in the sense of knowledge extraction tackles only a very elementary aspect of that. ===Tools=== The following criteria can be used to categorize tools, which extract knowledge from natural language text. The following table characterizes some tools for Knowledge Extraction from natural language sources. ==Knowledge discovery== Knowledge discovery describes the process of automatically searching large volumes of data for patterns that can be considered knowledge about the data. It is often described as deriving knowledge from the input data. Knowledge discovery developed out of the data mining domain, and is closely related to it both in terms of methodology and terminology. The most well-known branch of data mining is knowledge discovery, also known as knowledge discovery in databases (KDD). Just as many other forms of knowledge discovery it creates abstractions of the input data. The knowledge obtained through the process may become additional data that can be used for further usage and discovery. Often the outcomes from knowledge discovery are not actionable, techniques like domain driven data mining, aims to discover and deliver actionable knowledge and insights. Another promising application of knowledge discovery is in the area of software modernization, weakness discovery and compliance which involves understanding existing software artifacts. This process is related to a concept of reverse engineering. Usually the knowledge obtained from existing software is presented in the form of models to which specific queries can be made when necessary. An entity relationship is a frequent format of representing knowledge obtained from existing software. Object Management Group (OMG) developed the specification Knowledge Discovery Metamodel (KDM) which defines an ontology for the software assets and their relationships for the purpose of performing knowledge discovery in existing code. Knowledge discovery from existing software systems, also known as software mining, is closely related to data mining, since existing software artifacts contain enormous value for risk management and business value, key for the evaluation and evolution of software systems. Instead of mining individual data sets, software mining focuses on metadata, such as process flows (e.g. data flows, control flows, & call maps), architecture, database schemas, and business rules/terms/process. ===Input data=== Databases Relational data Database Document warehouse Data warehouse Software Source code Configuration files Build scripts Text Concept mining Graphs Molecule mining Sequences Data stream mining Learning from time-varying data streams under concept drift Web ===Output formats=== Data model Metadata Metamodels Ontology Knowledge representation Knowledge tags Business rule Knowledge Discovery Metamodel (KDM) Business Process Modeling Notation (BPMN) Intermediate representation Resource Description Framework (RDF) Software metrics ==See also== Cluster analysis Data archaeology ==References==

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