Dear Stat Talkers!

I'm using the following Stata command (see help mean):

mean x, std(a) stdw(b) over(c)

to get age-standardised mean or proportion of a variable of interest x.
a= variable for age-group and b= variable for weights for each
group. x can be a continuous variable or a binomial variable.

My question is how does Stata calculate 95% CI for such direct
age-standardised means/proportions? What method/approximation is used?

I tried to look up relevant help files under Methods but couldnt
really understand. Please help.

Many Thanks
Amit