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I have a doubt related to revised control limits for X bar R chart. For a given set of 10 observations, averages and range is given. Sample per observation is given as 4. I calc the UCL, CL and LCL for X bar chart and R Chart and construct the same. I find 2 points are out of UCL or LCL for x bar chart. Assuming that Observation 3 and Observation 7 are lying outside the control limits.
Now I am aware when I calculate the revised control limits, i need to exclude the observation 3 and 7 and calculate the revised X double bar to calc the UCL and LCL for remaining 8 observations. Does R bar get affected as well? Do I need to calculate R bar for the remaining 8 points as well.
Also if R bar chart has some points out of UCL and LCL, how do i calculate the revised control limits.?
Not to get too picky, but 10 points is hardly enough to be making a useful control chart. Plus, you just don’t exclude points because they are outside the control limits without a good deal of investigation to make sure the causes are assignable and actionable. As to the heart of your question, yes, you need to recalculate the R bar as well if you exclude the Xbar values. If the R bar has points out the Xbar chart has limited meaning. You must first control the process with respect to its variation. So, deal with the Rbar first and once that is OK, you can look at the Xbar chart.
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