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

Group of answer choices

13.0

4.33

5.4

4.2

2.0