Applications , page 4
Exercise 3 continued: x bar s chart. Figure 17 below shows an x bar chart created with the Excel spreadsheet charting tool, using the data in figure16, columns I through P. The x-axis labels were drawn from column A, although the date column or time column could be used as well.

Solution: Point number 23 lies near the lower control limit. The conditions producing point 23 should be checked. The s chart shows no excessive variation for this point. Point 23 appears to represent the occasional conjunction of low measures. No cyclic or other patterns are visible in the overall chart except for a possible downward trend. No runs of 7 or more points all increasing or decreasing can be seen. Runs of seven points all above of below the centerline are absent. There is no reason for corrective action to reduce special causes in this process. The s chart also shows a process in statistical control.
Figure 17. x bar chart created with a spreadsheet charting function.
The upper and lower control limits are shaded.
Figure 18. s chart created with a spreadsheet charting function.
The upper and lower control limits are shaded.
Statistics and SPC: 1 2 3 4 5 6 7
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