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 Measurements #1 17 14 15 18 #2 15 25 18 26 #3 23 25 23 21 What is the sample average range based upon this limited sample? 4.0 6.33 7.4 8.0 8.33