Okapi at TREC-4

The BSS, which has been used in all City's TREC experiments, is a set-oriented ranked output system designed primarily for probabilistic{type retrieval of textual material using inverted indexes. There is a family of built-in weighting functions as described in [1, Section 3]. In addition to weighting and ranking facilities it has the usual boolean and quasi-boolean (positional) operations. There are a number of non-standard set operations, some of which were added during the TREC{4 experiments.

[1]  Stephen E. Robertson,et al.  Relevance weighting of search terms , 1976, J. Am. Soc. Inf. Sci..

[2]  Stephen E. Robertson,et al.  On Term Selection for Query Expansion , 1991, J. Documentation.