Using JMP to Do Shewhart x and s Control Charting for Chapter 12 of MMD&S
Consider first the use of JMP to handle the "standards given" (known process parameters µ and σ )
charting of Section 12.1. We'll use JMP to take the data of Table 12.1 and make the charts shown in
Figures 12.4 and 12.7 (based on the limits shown in the box on page 12-18). The first "difficulty" is
that the data provided on the book's CD or Web site isn't set up properly to use JMP directly without a
bit of messing around. Do the following:
- Read in the JMP data for Table 12.1
- Choose "Stack" from the "Tables" menu and fill in the dialogue box as below
- Click on "Stack"
Now you have a worksheet with the 4 tension measurements for each sample listed immediately above
and below each other column-wise. Rename the column JMP has called "Stack" to "Tension." If you
like, you can save this new worksheet under a name of your choosing.
Note that the exposition of MMD&S proposes standard values of process parameters
µ = 275 and σ = 43
Select "Control Chart" from the "Graph" menu. Choose the option "Specify Stats" in the dialogue box
and fill the rest in as below.
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Clicking on "OK" should produce the desired control charts.
Now suppose that the authors had not furnished you with "All OK"/standard values for the process
mean and standard deviation (no one gives you µ and σ ). It is nevertheless possible to make
"retrospective"/"as past data" control charts that will allow you to ask the question "Is it plausible that
the process was stable at some set of process parameters?" (possibly different from the set
µ = 275 and σ = 43 ). Simply once again select "Control Chart" from the "Graph" menu, and this time
filling the dialogue box without choosing the "Specify Stats" option. Your filled-in dialogue box
should look like the one below.
Clicking on "OK" will produce control charts that are somewhat different than the "standards given"
charts based on µ = 275 and σ = 43 . The limits are from the "retrospective" formulas given in the box
on page 12-32. Notice that for the 20 samples of size n = 4 given in table 12.1, the sample standard
deviations average to
1
s = ( 21.8 + 33.0 + L + 38.8) = 34.56
20
so computing as on page 12-33 of the text (using c4 = .9213 for this sample size … see Table 12.3 on
page 12-19 for this)
34.56
σˆ =
= 37.51
.9213
This value is smaller than the earlier σ = 43 value, so the limits on both the x and s charts are tighter
than they were using the given standard values given earlier.
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