Whenever you type some search term in your Windows search box, some documents appear matching your search term. There is a common, well-known, easy-to-implement algorithm that makes it possible to rank the documents based on the search term. Basically, the algorithm allows developers to assign some kind of score to each document in the result set. That score can be seen as a score of confidence that the system has on how much the user would like that result.

The score that this algorithm attaches with each document is a product of two different scores. The first one is called term frequency (tf) and the other one is called inverse document frequency (idf). Their product is referred to as tf-idf or "term frequency inverse document frequency".

Tf is the number of times a term occurs in a given document. Idf is the ratio between the total number of documents scanned and the number of documents in which a given search term is found. However, this ratio is not...