Re: Q:Aggregrating Weekly Production Data. How do you do it?

Ow Mun Heng wrote:

> The results are valid (verified with actual data) but I don't  
> understand
> the logic. All the Statistical books I've read marked stdev as sqrt
> (sum(x - ave(x))^2 / (n - 1). The formula is very different, hence the
> confusion.

A formula is not an algorithm.  In particular, the naive way of  
calculating variance or standard deviation has massive numerical  
instability problems - anything involving sums of squares does.   
There are a variety of alternate algorithms for stddev/variance, I  
presume your other algorithm is similarly trying to avoid these same  
issues (but I have not looked closely at it).  You can also see  
Wikipedia for one of the most well known, due to Knuth/Wellford:

   http://en.wikipedia.org/wiki/Algorithms_for_calculating_variance

- John D. Burger
   MITRE