The KDD 2014 article, written by Dong, Gabrilovich, Heitz, Horn, Lao, Murphy, Strohmann, Sun, and Zang, describes in detail Google’s knowledge database Knowledge Vault. Knowledge Vault is a probability triple store database. Each entry in the database is of the form subject-predicate-object-probability restricted to about 4500 predicates such as “born in”, “married to”, “held at”, or “authored by”. The database was built by combining the knowledge base Freebase with the Wikipedia and approximately one billion web pages. Knowledge Vault appears to be the largest knowledge base ever created. Dong et al. compared Knowledge Vault to NELL, YAGO2, Freebase, and the related project Knowledge Graph. (Knowledge Vault is probabilistic and contains many facts with less than 50% certainty. Knowledge Graph consists of high confidence knowledge.)

The information from the Wikipedia and the Web was extracted using standard natural language processing (NLP) tools including: “named entity recognition, part of speech tagging, dependency parsing, co-reference resolution (within each document), and entity linkage (which maps mentions of proper nouns and their co-references to the corresponding entities in the KB).” The text in these sources is mined using “distance supervision” (see Mintz, Bills, Snow, and Jurafksy “Distant Supervision for relation extraction without labeled data” 2009). Probabilities for each triple store are calculated using logistic regression (via MapReduce). Further information is extracted from internet tables (over 570 million tables) using the techniques in “Recovering semantics of tables on the web” by Ventis, Halevy, Madhavan, Pasca, Shen, Wu, Miao, and Wi 2012.

The facts extracted using the various extraction techniques are fused with logistic regression and boosted decision stumps (see “How boosting the margin can also boost classifier complexity” by Reyzin and Schapire 2006). Implications of the extracted knowledge are created using two techniques: the path ranking algorithm and a modified tensor decomposition.

The path ranking algorithm (see “Random walk inference and learning in a large scale knowledge base” by Lao, Mitchell, and Cohen 2011) can guess that if two people parent the same child, then it is likely that they are married. Several other examples of inferences derived from path ranking are provided in table 3 of the paper.

Tensor decomposition is just a generalization of singular value decomposition, a well-known machine learning technique. The authors used a “more powerful” modified version of tensor decomposition to derive additional facts. (See “Reasoning with Neural Tensor Networks for Knowledge Base Completion” by Socher, Chen, Manning, and Ng 2013.)

The article is very detailed and provides extensive references to knowledge base construction techniques. It, along with the references, can serve as a great introduction to modern knowledge engineering.