Term Frequency - Inverse Document Frequency (TD/IDF)

Basic Tutorial on Information Retrieval
Entry level university class tutorial that assumes some mathematical familiarity, but which includes a fairly clear explanation of TF/IDF.

Understanding Inverse Document Frequency
Paper from Microsoft Research, Cambridge, authored by Stephen Robertson. (PDF Document)

Using TF-IDF to Determine Word Relevance in Document Queries
Paper at Rutgers University, authored by Juan Ramos. (PDF - 4 pages)

Query Expansion for Short Queries by Mining User Logs
Paper examines short vs. long user search queries and discusses expansion techniques, including tf*idf (Term Frequency & Inverse Document Frequency.
From Microsoft Research.

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