This time I will take the opportunity to talk about how to calculate the `STANDARD DEVIATIONS`

functions. The general formula for calculating the STDEV value for population is

As you can see in it’s original format, the function is a two-pass operation. First calculate the average value and the subtract that average value from each row in the current dataset.

I don’t find that good in terms of performance, so I figured out how to rewrite the function to be a one pass operation.

Can we apply the same thinking again when calculating the STDEV value for sample? Yes, we can.

Now we have a one pass operation and we can test the formula like this

The interesting thing here is that if you performance test my rebuild against the built-in Microsoft function, the rebuild is 25-30 percent faster! So it seems that the built-in function is using the two-pass operation. If Microsoft would adopt the rebuild, the function would be even faster and since it is non-blocking it also would be a candidate for batch mode operations.