Dr Wheeler weighs in on "Individual Charts Done Right and Wrong"

bobdoering

bobdoering

Stop X-bar/R Madness!!
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Dr Wheeler's article discusses the calculations for XmR charting:Individual Charts Done Right and Wrong He notes that some software will allow you to pick algorithms that calculate control limits erroneously. My favorite quote from this article: "Thus, just as with average charts, there are right and wrong ways to compute limits for a chart for individual values. Because most software will provide you with the option of computing your limits incorrectly, it is your responsibility to determine how to get the right limits using your software." I might add for some charting, the correct control limit methodology is not available in the software at all, and you are on your own.
 
Stijloor

Stijloor

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Super Moderator
bobdoering said:
Dr Wheeler's article discusses the calculations for XmR charting:Individual Charts Done Right and Wrong He notes that some software will allow you to pick algorithms that calculate control limits erroneously. My favorite quote from this article: "Thus, just as with average charts, there are right and wrong ways to compute limits for a chart for individual values. Because most software will provide you with the option of computing your limits incorrectly, it is your responsibility to determine how to get the right limits using your software." I might add for some charting, the correct control limit methodology is not available in the software at all, and you are on your own.
Click to expand...

Great article Bob! Thanks for sharing.

Stijloor.
 
Steve Prevette

Steve Prevette

Deming Disciple
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I do think that the "problems" with using the statistical estimate of the standard deviation need to be revisited. Some things to consider:

1. The example provided by Dr. Wheeler has a run of 7 points in a row below average, which by many authors (but not Dr. Wheeler, who uses 8 points) to be a signal, which would cause that data not to be put all into the same baseline.

2. In 16 years of operational SPC, I have yet to run into an example from real world data where the moving range and the statistical standard deviation provide two different interpretations of what is and what is not an outlier.

3. With modern spreadsheets, the statistical standard deviation is easier to calculate rather than the moving range conversion. That is the opposite of the situation for pre-computer days, when the moving range was developed.

4. Dr. Shewhart himself, in Economic Control of Quality of Manufactured Product evaluates several methods for determining the spread of the data, and he rates the statistical standard deviation as best (better than the range).

5. Dr. Shewhart and other authors do invoke the Tchybychev Inequality as the theoretical basis for SPC. The Tchybychev Inequality is non-parametric and uses the statistical estimate for the standard deviation. The moving range formula (2.66 times the average moving range) has the Normal distribution built into it as an assumption.
 
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