A quality analyst wants to construct a control chart for determining whether three machines, all producing the same product, are in control with regard to a particular quality variable. Accordingly, he sampled four units of output from each machine, with the following results:

Machine Measurements
#1 17 15 15 17
#2 16 25 18 25
#3 23 24 23 22

For upper and lower control limits of 23.29 and 16.71, which machine(s), if any, appear(s) to have an out-of-control process mean?

a. machine #1
b. machine #2
c. machine #3
d. all of the machines
e. none of the machines