Hi Valentin,
- What is bg_count (I assume background count) but what is the meaning of
it?
The bg_count is the number of documents, which contain the term in the
whole index (not just in the search result).
http://www.elasticsearch.org/guide/en/elasticsearch/reference/current/search-aggregations-bucket-significantterms-aggregation.html
- At first I thought the score values are between 0 and 1 but there are
much bigger values. Can anyone give me a rough explanation?
You can see the code of the computation here:
https://github.com/elasticsearch/elasticsearch/blob/master/src/main/java/org/elasticsearch/search/aggregations/bucket/significant/InternalSignificantTerms.java#L94
This is a summarized version of the formula:
double subsetProb = #relative frequency in the search result#;
double supersetProb = #relative frequency in the whole index#;
double absoluteProbChange = subsetProb - supersetProb;
if (absoluteProbChange <= 0) {
return 0;
}
double relativeProbChange = (subsetProb / supersetProb);
return absoluteProbChange * relativeProbChange;
I guess in the future there will be support for other scorings like
mutual information, chi squared or information gain.
Best regards,
Hannes
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