When n is large, the statistic s is approximately unbiased for estimating s and has approximately a normal distribution. The standard deviation of this statistic when the population distribution is normal is ss < s

!2n which can be estimated by s

!2n

. A large-sample confidence interval for the population standard deviation s is then s 6 1z critical value2 s

!2n Use the data of Example 9.9 to obtain a 95% confidence interval for the true standard deviation of waiting time for angiography.