Short introduction to Vector Space Model (VSM) In information retrieval or text mining, the term frequency - inverse document frequency also called tf-idf, is
S. Jespersen, T. Pedersen, and J. Thorhauge. Proceedings of the 5th ACM International Workshop on Web Information and Data Management, page 82--89. New York, NY, USA, ACM, (2003)
S. Lohmann, J. Kaltz, and J. Ziegler. Models in Software Engineering, volume 4364 of Lecture Notes in Computer Science, Springer Berlin Heidelberg, (2007)