Score-calculation.md

-## How the scores for a document is calculated
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+## Score calculation
+
+Source: https://www.compose.com/articles/how-scoring-works-in-elasticsearch/
+
+$`score(q, d) = queryNorm(q) * coord(q, d) * SUM(tf(t), idf(t)^2, t.getBoost(), norm(t, d)) (t in q)`$
+
+$`score(q, d)`$: The relevance score of a document $`d`$ for a query $`q`$ with terms $`t`$
+
+$`queryNorm(q)`$: The query normalization factor. Is used so that multiple different queries can be compared.
+
+$`coord(q, d)`$: The coordination factor. If a document contains 3 query terms its ranked higher than a document that only contains 2 terms. (Turn off for synonyms)
+
+Then the sum for each of the terms $`t`$ in $`q`$:
+- Query Boost: Combined with the queryNorm in explaination. Individual queries can be positively and negatively boosted when multple queries are executed.
+- Term frequency (tf): $`tf = sqrt(termFreq)`$
+- Inverse Doc Frequency (idf): $`idf = 1 + ln(maxDocs/(docFreq + 1))`$
+- Field length normalization (norm): $`norm = 1/sqrt(numFieldTerms)`$ Inverse square root of the number of terms in the field. Normalizes documents, so that long documents with many terms are scored like short documents with less terms. 
+
+### Field Boost
+One can add a higher weight to a field , so that this field influences the overall score more than other fields.
+```json
+"fields": [     "_content.Page.Content.Title.~^2", 
+"_content.Page.Content.Body.~"]
+```
+