What is Inverse Document Frequency (IDF)?
Inverse Document Frequency (IDF) is a weight indicating how commonly a word is used. The more frequent its usage across documents, the lower its score. The lower the score, the less important the word becomes.
For example, the word the appears in almost all English texts and would thus have a very low IDF score as it carries very little “topic” information. In contrast, if you take the word coffee, while it is common, it’s not used as widely as the word the. Thus, coffee would have a higher IDF score than the. Traditionally IDF is computed as:
where N is the total