override this if there is a faster way to do this sum than reduceLeftOption on plus
override this if there is a faster way to do this sum than reduceLeftOption on plus
uses a more stable online algorithm which should be suitable for large numbers of records similar to: http://en.
uses a more stable online algorithm which should be suitable for large numbers of records similar to: http://en.wikipedia.org/wiki/Algorithms_for_calculating_variance#Parallel_algorithm