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https://issues.apache.org/jira/browse/KYLIN-3361?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Xiaoxiang Yu reopened KYLIN-3361:
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> Support stddev as a new measure
> -------------------------------
>
> Key: KYLIN-3361
> URL: https://issues.apache.org/jira/browse/KYLIN-3361
> Project: Kylin
> Issue Type: New Feature
> Reporter: Zhong Yanghong
> Assignee: Zhong Yanghong
> Priority: Major
> Fix For: v3.1.0
>
>
> (x ~1~ - +x+) ^2^ + (x ~2~ - +x+) ^2^ + ... + (x ~n~ - +x+) ^2^ = x ~1~ ^2^ +
> x ~2~ ^2^ + ... + x ~n~ ^2^ - n +x+ ^2^, where +x+ is the average of x ~1~, x
> ~2~, ..., x ~n~. Therefore, to compute stddev, what kylin need to do is to
> pre-calculate sum(x ~i~ ^2^), sum(x ~i~) and count
>
> var(X) = E(X ^2^) - E(X) ^2^
> var ^'^(X) = n * var(X)
> = n*(E(X ^2^) - E(X) ^2^)
> = S(X ^2^) - S(X) ^2^/n
> = S(X ~1~ ^2^) + S(X ~2~ ^2^) - S(X ~1~ + X ~2~) ^2^/(n ~1~ + n
> ~2~)
> = S(X ~1~ ^2^) - S(X ~1~ ) ^2^/(n ~1~) + S(X ~2~ ^2^) - S(X ~2~)
> ^2^/(n ~2~) + S(X ~1~) ^2^/(n ~1~) + S(X ~2~) ^2^/(n ~2~) - S(X ~1~ + X ~2~)
> ^2^/(n ~1~ + n ~2~)
> = var ^'^(X ~1~) + var ^'^(X ~2~) + (S(X ~2~)n ~1~ - S(X ~1~)n
> ~2~) ^2^ / (n ~1~ n ~2~ (n ~1~ + n ~2~))
> = var ^'^(X ~1~) + var ^'^(X ~2~) + (S(X ~2~) - S(X ~1~) n ~2~ /
> n ~1~ ) ^2^ n ~1~ / (n ~2~ (n ~1~ + n ~2~))
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