Vol. 8, No. 1

January 2006

PQ Systems

Easy online training

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.

Phillip D. Tank knows how to create an X-bar and R chart, but perhaps he doesn't understand how to interpret one. Control chart interpretation is a process, and a flow chart illustrates the ways in which analysis can take place in a step-by-step manner:

This flow chart is taken from Practical Tools, page 200. Phillip's chart has no points outside the control limits, and there seem to be no runs. But the final test, "Nonrandom patterns present?" has clearly been violated in Phillip's chart:

In viewing the chart, you can see a large number of X-bars close to the upper control limit and the lower control limit. This indicates a "non-random pattern" is present.

On the other hand, if you have a process that is in control you will get a chart like the one below. As can be seen, there is no cluster of X-bars close to the upper and lower control limits.

If we took the X-bars from this chart and created a histogram, then it would be normal looking. The upper control limit would be three standard errors above the process average and the lower control limit would be three standard errors below the process average.

The below-normal distribution shows that going plus and minus two standard errors will capture about 95 percent of the area under the curve. Therefore, the area beyond plus and minus two must be 5 percent.

Phillip's chart, on the other hand, includes far more than 5 percent-an indication of an out-of-control process. This is a clear indication that a non-normal pattern is present.

It looks like Phillip needs to do some more reading on control chart interpretation.

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