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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 What is the sample mean for machine #12 O 16 23 21 15 17