How many bits are needed to store term frequencies?

Search algorithms in most current text retrieval systems use index data structures extracted from the original text documents. In this paper we focus on reducing the size of the indices by reducing the amount of space dedicated to store term frequencies. In experiments using TREC Ad Hoc [2, 3] corpora and query sets, we show that it is possible to store the term frequency in only two bits without decreasing retrieval performance.