>I'm in a bit of a problem here. I need to compute the standard deviation on an array of values. I know it's easy to do it on table data: CALCULATE STD(numfield). But what is the actual textbook formula? Anyone got a mathematics textbook handy or a darn good memory?

I see Mark found you the formula - careful about the squared, it's not on the entire summation, but on each individual x(i) - Mean term. You have to make two passes throught the data for this, of course. There is a slightly easier computational version of this as follows:

SD = SQRT(n * sum(x(i)^2) - sum(x(i))^2/(n * (n-1))

That said, I would use JVP's suggestion of dumping it into a cursor, much nicer.