On the Necessity of Term Dependence in a Query Space for Weighted Retrieval

In recent years, in the context of the vector space model, the view, held by many researchers, that documents, queries, terms, etc. are all elements of a common space has been challenged (Bollmann-Sdorra and Raghavan, 1993). In particular, it was noted that term independence has to be investigated in the context of user preferences and it was shown, through counter examples, that term independence can hold in the document space, but not in the query space and vice-versa. In this paper, we continue the investigation of query and document spaces with respect to the property of term independence. We prove, under realistic assumptions, that requiring term independence to hold in the query space is inconsistent with the goal of achieving better performance by means of weighted retrieval. The result that term independence in the query space is undesirable is obtained without making any assumption about whether or not the property of term independence holds in the document space. The results of this paper reinforce our position that the properties of document and query spaces must be investigated separately, since the document and query spaces do not necessarily have the same properties.