Example of C chart with customized control limits
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Suppose you work for a linen manufacturer. Each 100 square yards of fabric can contain a certain number of blemishes before it is rejected. For quality purposes, you want to track the number of blemishes per 100 square yards over a period of several days, to see if your process is behaving predictably. You want the control chart to show control limits at 1, 2, and 3 standard deviations above and below the center line.

1    Open the worksheet EXH_QC.MTW.

2    Choose Stat > Control Charts > Attributes Charts > C.

3    In Variables, enter Blemish.

4    Click C Chart Options, then click the Limits tab.

5    Under Display additional sigma limits at, enter 1 2 in These multiples of the standard deviation.

6    Under Place bounds on control limits, check Lower standard deviation limit bound and enter 0.

7    Click OK in each dialog box.

Graph window output  

Interpreting the results

Because the points fall in a random pattern, within the bounds of the 3s control limits, you conclude the process is behaving predictably and is in control.