1. A Quality Analyst wants to construct a control chart for determining whether three machines, all producing the same product, are under control with regard to a particular quality variable. Accordingly, he sampled four units of output from each machine, with the following results : Machine #1 measurements [21, 18, 21, 12]; Machine #2 measurements [ 13, 24, 21, 17]; Machine #3 measurements [ 24, 20, 24, 20]. Using the factors for three sigma control limits, what are x-bar chart upper and lower control limits?

a. 25.42and 13.74

b.43.58and -4.417

c.21.74and 17.42

d.27.74and 11.42