I just saw the answer by Mike. I like the more compact way to get the multiplication factor (CHECKSUM_AGG), but I'm also a bit concerned - I could not find any documentation (at least not in the 30 seconds I spent searching -I know, pathetic!-) that this aggregate guarantees a negative result if the number of negative input values is odd, and a positive result otherwise. Without such documentation, this method appears to be less safe.

And a final comment regarding the original article that I forgot in my previous post:

"Notice the decimal point after the first argument of the power function. You need it to force a precession of 18 so that 4 * 2 equal 8 instead of 7."

The decimal point does not force a precision of 18, it forces the use of non-integer (numeric(5,2) to be precise). Without this, all numbers after the decimal place are cut off. That's why most versions of this algorithm on the internet use natural logarithm rather than 10-based logarithm - that one always uses non-integer calculation.