GitHub user rustamhsmv opened a pull request:

https://github.com/apache/lucene-solr/pull/155

SOLR-10082 : Add Variance and Standard Deviation aggregators

You can merge this pull request into a Git repository by running:

$ git pull https://github.com/rustamhsmv/lucene-solr VarianceStddev

Alternatively you can review and apply these changes as the patch at:

https://github.com/apache/lucene-solr/pull/155.patch

To close this pull request, make a commit to your master/trunk branch
with (at least) the following in the commit message:

This closes #155

commit 6e1e1ac0296400bd87def4fa62ebab9541c1f9a4
Author: rustamhsmv <rustam.hashimov@outlook.com>
Date: 2017-01-31T22:34:05Z

Add Variance and Stddev Aggregators

commit 672138bf34bb7eee43add03e5d04aaff25955d17
Author: rustamhsmv <rustam.hashimov@outlook.com>
Date: 2017-01-31T22:40:20Z

Merge branch 'master' of https://github.com/rustamhsmv/lucene-solr into VarianceStddev

I'm taking a look at this now...
Regarding the rounding up of variance and standard deviation to the nearest 10th... I'm inclined to remove it since it would seem to be dependent on the domain?

further, it doesn't look like the code would actually round up as the comments describe:

return Math.round(val * 10) / 10.0; // round up to 1 decimal place

After removing the rounding, I'm seeing much larger variations than I would expect for distrib vs non-distrib. As an example:
'74.7'!='74.66666666666667'

At this point, I'm not sure if the numerical methods used here are that unstable (squaring big things can tend to swamp smaller contributions), or if there is another bug somewhere that the rounding was obscuring.

Commit 3145f781b3b55ebbd6374f40db7fe7b676b8d1af in lucene-solr's branch refs/heads/master from Yonik Seeley
[ https://git-wip-us.apache.org/repos/asf?p=lucene-solr.git;h=3145f78 ]

SOLR-10082: JSON Facet API, add stddev and variance functions

Commit 46012784d4999790190a8d6c3d8cc697f7541160 in lucene-solr's branch refs/heads/branch_6x from Yonik Seeley
[ https://git-wip-us.apache.org/repos/asf?p=lucene-solr.git;h=4601278 ]

SOLR-10082: JSON Facet API, add stddev and variance functions

OK, I found the issues (I missed some places that were still rounding) and committed. Thanks Rustam!