Chapter 7.4, Problem 16EC

Calculator Battery Lifetimes A confidence interval for a standard deviation for large samples taken from a normally distributed population can be approximated by

$s-z_{\alpha }/2\frac{s}{\sqrt{2n}}<\sigma <s+z_{\alpha }/2\frac{s}{\sqrt{2n}}$

Find the 95% confidence interval for the population standard deviation of calculator batteries. A random sample of 200 calculator batteries has a standard deviation of 18 months.