Een samenvatting van alle lectures (1 t/m 12) van het vak Web Data Processing Systems aan de VU Amsterdam. Kort, duidelijk en overzichtelijk samengevat met ondersteundende afbeeldingen waar nodig.
Knowledge bases
First Information Retrieval was based on keywords. Now it is based on entities.
Symbolic Knowledge Bases (KBs)
● Meaning accessible to humans
● Constructed manually or from unstructured sources
● Can be expressed using first-order logic (knowledge graphs):
Latent Models
● Meaning is hidden
● Learned using machine learning techniques
● Prominent example: Google’s word2vec
RDF (Resource Description Framework)
● Standard used to report statements that describe properties of resources
● Statements can be represented as triplets of the form <s p o> (subject predicate object) and
serialized with different formats (RDF/XML, N3, Turtle)
● RDF dataset can be represented as a directed graph
● SPARQL is used to query RDF databases (inspired by SQL)
○ Finding answers to a query corresponds to finding all possible graph homomorphisms
between the query and the graph
Knowledge bases on the web
WordNet
● Groups words into sets of synonyms called synets.
● Words can be monosemous (one meaning) or polysemous (multiple meanings)
● Each synet has a gloss (short description) and is connected to other synets using relations. Most
important:
○ Hypernyms/Hyponums (isA)
○ Meronym/Holonyms (partOf)
DBpedia
● Project to convert Wikipedia pages to RDF
● Uses structured data on the pages
● Contains links to other KBs (widely popular in the “linked-data-cloud”
● Fairly large ontology but not rich in terms of expressiveness
● Alignment between infoboxes and ontologies is done via community-provided mappings
Yago (Yet another great ontology)
● Goals:
○ Unify Wikipedia and Wordnet
, ○ Extract clean facts
○ Check plausibility of facts via type checking
● High standard in terms of quality
Freebase
● Collaborative knowledge base by its community
● Acquired by Google, but shutdown in 2014
Wikidata
● Mainly text → hard to verify and keep consistency
● “Data version” of Wikipedia
○ Validated by community
○ Keeps provenance of the data
○ Multilingual
○ Supports plurality
● High quality knowledge
Natural Language Processing (NLP)
Knowledge acquisition: process to extract knowledge (to be integrated
into knowledge bases) from unstructured text or other data
Preprocessing
Tokenization
Split sequence into tokens (terms/words)
● Token: instance of a sequence of characters in some particular document that are grouped
together as a useful semantic unit
● Type: class of all tokens containing the same character sequence
● Example: “A rose is a rose is a rose”
○ Tokens: 8
○ Types: 3 ({a, is, rose})
Queries and documents have to be preprocessed identically. It determines which queries match.
Problems:
● Hyphens (Co-education, drag-and-drop)
● Names (San Francisco, Los Angeles)
● Language (compound nouns in German v.s. separate nouns in English)
Lemmatization
Goal: reduce words to base form (Lemma; as defined in dictionary)
, ● Am, are, be, is → be
● Car, cars, car’s, cars’ → car
Stemming
Goal: reduce words to their “roots”
● Are → ar
● Automate, automates, automatic, automation → automat
Stop word removal
Based on a stop list, remove all stop words. All words that are not part of the IR system’s dictionary.
● Saves memory
● Makes query processing faster
Part-of-speech (POS)
Assign a label to each token that indicates what the function is in the context.
● Function words: used to make sentences grammatically correct
○ Prepositions, conjunctions, pronouns, etc.
● Content words: used to carry the meaning of a sentence
○ Nouns, verbs, adjectives, adverbs
Part-of-speech tags allow for a higher degree of abstraction to estimate likelihoods.
How do they work?
● Rule-based taggers
● Stochastic taggers. Most used and rely on Hidden Markov Models. Based on likelihood.
Other NLP tasks
Parsing
Construct a tree that represents the syntactic structure of the string according to some grammars.
Constituency parsing
Breaks the phrase into sub-phrases. Nonterminals in the tree are types of phrases, the terminals are the
words in the sentence, and the edges are unlabeled.
Dependency parsing
Connect the words according to their relationships. Each vertex in the tree represents a word, child
nodes are words that are dependent on the parent, and edges are labeled
by the relationship.
Information Extraction
Two types of information extraction: Named Entity Recognition (NER) and Relation Extraction (RE).
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