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Colin,

Lack of STD is not a very high wall. No higher than 2 feet. You can easily jump over it.

Use:

SQRT(
Sum(x^2)/countrows() –
(sum(x)/countrows())^2
)

some use N-1 instead of N (i.e. countrows()-1)

Look for more details at
http://knol.google.com/k/suresh-emre/how-to-compute-standard-deviation-in/35vsnxisjn2mw/128#

(Hat tip to Amir Netz)