One of the most common types used in older retrieval models is known as tf.idf
weighting. There are many variations of these weights, but they are all based on a
combination of the frequency or count of index term occurrences in a document
(the term frequency, or tf ) and the frequency of index term occurrence over the
entire collection of documents (inverse document frequency, or idf ). The idf weight
is called inverse document frequency because it gives high weights to terms that
occur in very few documents. A typical formula for idf is logN/n, whereN is the total number of documents indexed by the search engine and n is the number of
documents that contain a particular term.