DETERMINING THE APPROPRIATE SAMPLE SIZE
1.
Sample size for Estimating 
. If 
is used as an
estimate of 
, we can be (1-
) 100% confident that the error will not exceed a specified
amount e when the sample size is
n = 
The formula is
applicable if we know s but we can use
the s from a preliminary sample that is n>30 for our estimate
of s. The e, 
,
, and s should have same measurement unit.
2.
Sample size for estimating p. If
is used as an estimate of p, then we can be (1-
) 100% confident that the error will not exceed a chosen e
when the sample size is
n = 
Note that the e here
is a proportion.
3.
Where it is not useful to pre-determine a
or an earlier estimate of 
is not available, use the alternative rule: If
is used as an estimate of p, we can be at least (1-
) 100% confident that the error will not exceed a chosen e
when the sample size is
n = 