variance aggregate function incorrect? Reference Materials reg create aggregate

hi
i needed the variance function ... i dont know if i introduced the wrong
parameters or maybe this variance is not the variance that im looking
for but it doesnt provide the right results

variance=(nEx^2 - (Ex)^2)/(n(n-1))
my sql statement
select variance(answer) from tbl_answer (where answer is of type
integer)

because of this i need to create my own variance aggregate (unless
someone enlightens me on whats happening with the variance aggregate) so
i would like to know if theres anyone who knows a good create aggregate
tutorial (with samples) ... all i got from google so far is the
reference from ninthwonder.com
TIA