# General Formula for Confidence Intervals (1 of 2)

If a

statistic is

normally distributed and the

standard error of the statistic is known,
then a

confidence interval for that
statistic can be computed as follows:

statistic ± (z) (σ_{stat})

where σ

_{stat} is the standard error of the statistic. For instance,
the

confidence interval for the mean is:

M ± z σ_{M}.

where M is the sample mean and σ

_{M} is the standard error of the mean. If the
standard error has to be estimated then the formula uses

t:
statistic ± (t) (s_{stat})

where s

_{stat} is the estimated standard error of the statistic. For
example, the

confidence interval for the
mean when σ is estimated is

M ± t s_{M}.