[Yt-dev] sigma

[Matthew Turk]

> I can make it an upper triangular matrix multiply.

Okay, so that should work for decomposing across grids, right?  We
want to be able to cut this up and then do a single combine step at
the end, after each grid is handled.

> You want a weighted sum because the numpy.array.sum function is super
> optimized, over looping which is slow?

Well, sure, but more because what we're trying to do is:

sum ( w_i * v_i ) / sum (w_i)

so we'll need to weight the values to calculate std-dev, yes?



-Matt