Vol. 11, No. 11
November 2009
PQ Systems
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Quality Quiz from Professor Cleary

"C" is correct.

Click here for a more complete video explanation

This concept has everything to do with control charts. Once it has been determined that the distribution of sample means form a normal distribution, it is clear where the control limits come from. The figure below shows the upper and lower control limits as plus and minus three standard errors . If the process is stable (in control), one could expect to see 99.734% of the ’s to be within these limits.

If a point appears outside the limits, this indicates that the process is not in control. This, of course, is the purpose of control limits—to reflect whether a process is in or out of control.

Perhaps Hartford Simsack should return to his previous position, where he could do less damage.

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