What are the natural tolerance limits of this process? The thickness of a metal part is an important quality parameter. Data on thickness (in inches) are given here, for 25 samples of five parts each.
Sample Number123456789101112131415x10.06290.06300.06280.06340.06190.06130.06300.06280.06230.06310.06350.06230.06350.06450.0619x20.06360.06310.06310.06300.06280.06290.06390.06270.06260.06310.06300.06300.06310.06400.0644x30.06400.06220.06330.06310.06300.06340.06250.06220.06330.06330.06380.06300.06300.06310.0632x40.06350.06250.06330.06320.06190.06250.06290.06250.06300.06310.06350.06270.06300.06400.0622x50.06400.06270.06300.06330.06250.06280.06270.06270.06240.06300.06330.06290.06300.06420.0635
Sample Number16171819202122232425x10.06310.06160.06300.06360.06400.06280.06150.06300.06350.0623x20.06270.06230.06300.06310.06350.06250.06250.06320.06290.0629x30.06300.06310.06260.06290.06290.06160.06190.06300.06350.0630x40.06280.06200.06290.06350.06350.06200.06190.06310.06310.0626x50.06290.06250.06280.06340.06340.06230.06220.06300.06330.0628
(a) Using all the data, find trial control limits for X and R charts, construct the chart, and plot the data. Is the process in statistical control? (b) Use the trial control limits from part (a) to identify out-of control points. List the sample numbers of the out-of-control points. Continue eliminating points and revising control limits until the charts are based only on in-control observations.