Historically, it was very important in introducing these concepts, and effective techniques have been developed through years of experimentation. As a retrieval model, however, it has major flaws. Although it provides a convenient computational framework, it provides little guidance on the details of how weighting and ranking algorithms are related to relevance. In this model, documents and queries are assumed to be part of at-dimensional vector space, where t is the number of index terms (words, stems, phrases, etc.). A document Di is represented by a vector of index terms: