Applications , page 3
Exercise 3
Make an x bar s chart from 30 samples of hardness data from the default settings; show the process center and control limits. Make observations about the process, using the chart and the data.

Exercise 3 solution
Thirty samples size of 5 were taken, one sample from each batch of 288 blades. Figure 16 shows the data in columns A through H. At the top of the sheet are the control limit constants for samples of size 5. Columns I and J contain the x bar and s data to be plotted. Columns K through P hold copies of the control limits and averages to be plotted as horizontal lines. Control limit recalculations should be done after a change in the process-- changes in methods, materials, people, or machines. The process needs to be in control, that is fee of special causes, before control limits can be calculated. In a baseline calculation such as this, if there are any points out of control, they can be eliminated, the control limits calculated, and the process monitored for special causes of variation.
Figure 16. Hardness data with computations set up for charting using a spreadsheet.
Figure 16a. Hardness data spreadsheet formulas.
Figure 16a shows the formulas behind the values used to plot x bar and s charts using a spreadsheet. The only quantities that vary are the averages in column I and the standard deviations in column J. The values in K through P are the process center and the control limits which do not change. They will plot as straight lines. In Excel the marker feature must be deselected or the lines will look ragged. Cells E2, F2 and J2 hold control chart constants.
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