The authoritative document for scoring is found on the Lucene site here. Read that first.
Lucene implements a variant of the TfIdf scoring model. That is documented here.
The factors involved in Lucene's scoring algorithm are as follows:
The implementation, implication and rationales of factors 1,2, 3 and 4 in DefaultSimilarity.java, which is what you get if you don't explicitly specify a similarity, are:
note: the implication of these factors should be read as, "Everything else being equal, ... [implication]"
1. tf Implementation: sqrt(freq) Implication: the more frequent a term occurs in a document, the greater its score Rationale: documents which contains more of a term are generally more relevant 2. idf Implementation: log(numDocs/(docFreq+1)) + 1 Implication: the greater the occurrence of a term in different documents, the lower its score Rationale: common terms are less important than uncommon ones 3. coord Implementation: overlap / maxOverlap Implication: of the terms in the query, a document that contains more terms will have a higher score Rationale: self-explanatory 4. lengthNorm Implementation: 1/sqrt(numTerms) Implication: a term matched in fields with less terms have a higher score Rationale: a term in a field with less terms is more important than one with more
queryNorm is not related to the relevance of the document, but rather tries to make scores between different queries comparable. It is implemented as 1/sqrt(sumOfSquaredWeights)
So, in summary (quoting Mark Harwood from the mailing list),
* Documents containing *all* the search terms are good * Matches on rare words are better than for common words * Long documents are not as good as short ones * Documents which mention the search terms many times are good
The mathematical definition of the scoring can be found at http://lucene.apache.org/core/4_0_0/core/org/apache/lucene/search/similarities/Similarity.html
Hint: look at NutchSimilarity in Nutch to see an example of how web pages can be scored for relevance
Its easy to customize the scoring algorithm. Subclass DefaultSimilarity and override the method you want to customize.
For example, if you want to ignore how common a term appears across the index,
and if you think for the title field, more terms is better