Vol. 7, No. 08
Now that Marsha Mallow and Maura Thesame have learned how to create control charts using CHARTrunner, and how to calculate upper and lower control limits three standard errors () above and below the process average (), technicians are churning out control charts on the packaging lines for 12-ounce packages of cottage cheese.
Line #1 seems to be working fine, with a process that is definitely in control:
Line #2, on the other hand, reflects some averages above the upper control limit. A technician shows the chart to Marsha, who acknowledges that the chart is “very interesting” and requires some deeper analysis.
Huddling with Maura, the two go to their source, SPC Basics (Cleary and Graham), and discover a graphic that may give them insight about their response to the technician’s chart for Line #2. “Wow. What a fascinating graphic,” they exclaim.
The “in control” chart, they see, suggests that the process does not change over time. Its central location is the same, variability does not change, and the shape of the distribution remains constant. The “out of control” chart, on the other hand, is not characterized by the same constancy. Marsha observes that things seem to be changing over time, as they are on Line #2. Maura explains, however, that if the changes are all averaged, the process will continue to produce predictable outcomes.
Is this analysis deep enough to have derived the correct conclusion?
Please direct questions or problems regarding this web site to the Webmaster.