Quality Quiz from Professor Cleary
"No" is the correct answer!
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The correct answer is clearly “no.”
This chart reflects a run. As the following flow chart (from Practical Tools for Continuous Improvement, Cleary and Graham) indicates, when there are 7 or more points in a row above or below the center line, the process is unstable or out of control.
Marsha and Maura might have focused exclusively on the first test for stability: “Points outside the control limits?” without moving on to the next test.
The graphic (Figure A )indicates a process that is in control. Figure B demonstrates a change in the process that occurs beginning at 10:00, when the process shifts up, or to the right.
When this occurs, one would expect to get sample averages (X-bars) above the grand average (X double bar). That is what the chart that reflects production on Line #2 shows, indicating an out-of-control signal.
Of course, seeing a chart like this may make one feel, as the technician did, that something had changed. In the example, there are 10 X-bars above the X-double bar, so the chart is not just hinting, but screaming, that something has changed.
The question becomes one of determining how many points above X-double bar are required in order to declare a run, or to say that the process is out of control. While 7 is the common rule, others consider 8 as a determining number.
Next month, we will examine this situation from a statistical point of view.
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