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

#$117151517$

#$216251825$

#$323242322$

What is the estimate of the sample average range based upon this limited sample?

$2\mathrm{.}0$

$4\mathrm{.}33$

$5\mathrm{.}4$

$13\mathrm{.}0$

$4\mathrm{.}2$