In this discussion, we have yet to say anything about the form of the term
weighting used in the vector space model. In fact, many different weighting schemes
have been tried over the years. Most of these are variations on tf.idf weighting,
which was described briefly in Chapter 2. The term frequency component, tf, reflects
the importance of a term in a documentDi (or query). This is usually computed
as a normalized count of the term occurrences in a document, for example
by