Posted by Carolyn on .
A quality control inspector wants to estimate the mean inside diameter of the washers produced by a
certain machine. A preliminary sample indicates that the inside diameters have a standard deviation
of 0.30 millimeters. What sample size does he need to estimate the mean to within an error of E = 0.02
millimeters with 90% confidence?

Statistics 
PsyDAG,
90% = mean ± 1.645 SEm
SEm = SD/√n
.02 = 1.645 SEm