see https://gist.github.com/1886960 for an algorithm to handle calculating standard deviation and variance across many servers.
Pull request mongo/pull/409
I'm assuming this is meant to be for $group.
Message: Clean up AccumulatorAvg
Major change is not inheriting from AccumulatorSum
Prep for SERVER-5044 $stdDev accumulator
This is a must have.
Hey, It's a nice to have feature.
Any updates on this ?
Message: SERVER-5044 Standard Deviation accumulator operators
Message: SERVER-5044 Use relative error when testing sharded $stdDev operations