Vol. 7, No. 9

September 2005

PQ Systems

Software support: Helpful help

Quality Quiz: With a video!

Six Sigma

Data in everyday life

Bytes and pieces

FYI: Current releases


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Quality Quiz from Professor Cleary

"No" is the correct answer!

Click here for a more complete video explanation.

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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