example 4.14 Calculating a Confidence Interval for Variances For the coffee shop example, the raw data is given for the service times of 25 customers. Our previous standard deviation was 5 seconds. Develop 90% confidence intervals on upper side to determine if we have reduced the standard deviation with the new equipment purchase. Using this data set, the sample standard deviation is 4.008 . There are 25 data points, so there are 24 degrees of freedom. Since we are building a 90% confidence interval, alpha =10%