estimate±margin of error
The margin of error for a level C confidence interval for the mean μ of a Normal population with known standard deviation σ, based on an SRS of size n, is given by
m=z*σ√n
Here, z* is obtained from the row labeled z* at the bottom of Table D. The probability is C that a standard Normal random variable takes a value between -z* and z*. The level C confidence interval is
ˉx±m
If the population is not Normal and n is large, the confidence level of this interval is approximately correct.
n=(z*σm)2