Early IR systems were boolean systems which allowed users to specify their information need using a complex combination of boolean ANDs, ORs and NOTs. Boolean systems have several shortcomings, e.g., there is no inherent notion of document ranking, and it is very hard for a user to form a good search request. Even though boolean systems usually return matching documents in some order, e.g., ordered by date, or some other document feature, relevance ranking is often not critical in a boolean system. Even though it has been shown by the research community that boolean systems are less effective than ranked retrieval systems, many power users still use boolean systems as they feel more in control of the retrieval process. However, most everyday users of IR systems expect IR systems to do ranked retrieval. IR systems rank documents by their estimation of the usefulness of a document for a user query. Most IR systems assign a numeric score to every document and rank documents by this score. Several models have been proposed for this process. The three most used models in IR research are the vector space model, the probabilistic models, and the inference network model.