Inverse Document Frequency

Similar to the normalized Term Frequency we want to find out how much a given word $t$ is found in a collection of documents $D$.

$\mathrm{idf}(t,D):={\log }_{10}\left(\frac{N}{|\{d\in D\mid t\in d\}|}\right)$

Intuition This formula is near $0$ when a word is present in nearly all documents. For example, the word “the” will most likely have a value of zero. This will ensure that “meaningless” words will not be used for Information Retrieval.