TF-IDF

This measure, proposed by G. Salton (1989), allows us to evaluate the weight of a term (lexical unit) within a document (context unit).

Its formula is the following:

w i,j = tf i,j x idf i (Term Frequency x Inverse Document Frequency)

Where:

tf i,j = number of occurrences of i (term) in j (document)
df i = number of documents containing i
N = total number of documents

Term Frequency (tf i,j ) value can be normalized as follows:

tf i,j = tf i,j / Max (f i,j )

where Max (f i,j ) is the maximum frequency of i(any term) in the j (document).