Vol. 10, No. 11

November 2008

PQ Systems
 
Contents

Quality improvement in schools

Quality Quiz: With a video!

Data in everyday life

Six Sigma

Design of experiments

Bytes and pieces

FYI: Current releases

 

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Software

 

   

Quality Quiz from Professor Cleary

Congratulations:
"B" is correct.

Click here for a more complete video explanation

Indeed, Dr. Ryaction is wrong. The control limits of an individuals and moving range chart are determined by the mean of the moving range: If is inflated, the control limits on the individuals chart will be excessively wide. Some learning opportunities may be missed, since out-of-control points might be overlooked.


CHART 1

This chart again shows both the individual values and the moving range. It is clear that the moving range for point #23 is out of control. Since the moving range is outside the limits on the high side, it will cause the average moving range to be inflated, and in turn, to inflate the upper control limit. This could potentially mask an assignable cause.

In such circumstances, the Lloyd Nelson option should be applied. This feature, built into CHARTrunner, eliminates any data point if the moving range for that data point is out of control, then recalculates control limits. Using the Lloyd Nelson option, Arch N. Emee creates the chart again. The moving range for point #23 is eliminated and the upper and lower control limits recreated. (It should be noted that the individuals value for point #23 is not eliminated from the calculation of ).Now the chart indicates that point #22 is out of control on the high side. This situation was of course hidden before the out-of-control point of the moving range for point #23 was eliminated.


CHART 2

 

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