20.2.6. Hit Scores
When a document satisfies a text search expression a score is computed to reflect the closeness of the match. This is a positive integer, with a higher value meaning a closer match. The scores are only comparable between results of the same query against the same database. Moreover, the scoring rules are different for different versions of Virtuoso server, due to the progress in information retrieval theory. Thus the only really safe thing to do with scores is to sort the list of hits by descending score to produce more relevant hits first. In addition, the CONTAINS predicate allows specifying a lower limit to the score of produced hits so a smart application can reduce a number of retrieved documents by filtering out the less relevant documents.
While the real scoring rules of the server are too sophisticated to be listed here, the basic concept is simple: hits for restrictive terms are most important than hits for generic terms, frequent hits in same document are more important than occasional, hits that are probably concentrated in same sentence are more important than hits here and there in a long text:
|The score of a single word term is proportional to the frequency of the word in the document.|
|The score of a phrase is proportional to both the frequency of the phrase in the document and the number of words in the phrase.|
|The score of a proximity term 'A NEAR B' depends on the distance between A and B in words - greater distances result in smaller scores. E.g. the score is 100 for zero distance, 80 for 5 word distance, 50 for distance 10, 20 for distance 20; then it slowly decreases to 1 for distance 100. If the distance is 100 words or greater then the condition A NEAR B is considered not to be satisfied. If the A-B pair occurs several times throughout the document the scores of each pair are added. Virtuoso searches for shortest possible pairs, so if there is more than one word A around B then the nearest A is selected for scoring.|
|Score of a term like A AND B AND C... AND Z is a minimum of scores of A, B,... Z plus some bonuses. Longer list of AND'ed terms get higher score. A special bonus for term A AND B is added if hits of subterm A are close to the hits of subterm B.|
|Score of a term like A OR B OR C... OR Z is a maximum of scores of A, B,... Z, with some additional corrections. Longer list of OR'ed terms get lower score. A special bonus for term A OR B is added if hits of subterm A are close to the hits of subterm B.|
|Score of a term like A AND NOT B is equal to the score of subterm A. The subterm B is used for filtering out redundant hits but it does not affect the scoring.|
The XCONTAINS predicate can also return scores. These scores are somewhat similar to scores made by CONTAINS but rules for them are too complicated to be explained here.