Aquality 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 22what is the estimate of the sample average range based upon this limited sample?