It can be shown that the variances of 
and 
can be
approximated by[7] 
:
where 
and 
be the mean of the
fractions of the interval 
lived by persons who die in
the interval.
In case of life table use, the first term of equation 17:
is negligible in comparison with
the other term (because 
is much smaller [18]).
Generally 
can be approximated by:
where 
is the number of persons of age i participating to the
survey. So, equation 17 can be approximated by:
And for the same reasons, equation 18 can also be approximated by:
Remark: In some cases, it could be more
appropriate to take an other variance of 
than
equation 20, related to the special characteristics of the
sample used for the survey. In that case, reader needs to refer to the
details of the survey itself (for example, in case of a stratified
sample).
