The file P08_06.xlsx contains data on repetitive task times for each of two workers. John has been doing this task for months, whereas Fred has just started. Each time listed is the time (in seconds) to perform a routine task on an assembly line. The times shown are in chronological order.
a. Find a 95% confidence interval for the standard deviation of times for John. Do the same for Fred. What do these indicate?
b. Given that these times are listed chronologically, how useful are the confidence intervals in part a? Specifically, is there any evidence that the variation in times is changing through time for either of the two workers?