The overall length of a skew used in a knee replacement device is monitored using and R charts. The following table gives the length for 20 samples of size 4. (Measurements are coded from 2.00 mm; that is, 15 is 2.15 mm.)
(a) Using all the data, find trial control limits for and R charts, construct the chart, and plot the data.
(b) Use the trial control limits from part (a) to identify out of-control points. If necessary, revise your control limits, assuming that any samples that plot outside the control limits can be eliminated.