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Steve Wise

Six Sigma

Chart My Gauge

If you dare

Published: Tuesday, April 3, 2007 - 22:00

Some folks like to use control charts to analyze gauge study results. When using a control chart in this manner, one can assume that the chart should represent a series of gauge studies conducted over a period of time. In this example, let’s say that each morning a gauge is verified by recording five repeated measurements of a certified gauge block. Of course, the variation within the five readings would indicate the gauge’s repeatability, and the stability of the gauge would be exhibited on the control chart. What’s the best control chart to use to monitor this gauge?

    1. A Target Group chart, because the readings are easier to interpret when grouping all the studies and displaying the results as a deviation from the known reference value.
    2. A Traditional Xbar and R chart using economic control limits because, given this sampling strategy, the plot points on the Xbar chart are going to be woefully out of control. Therefore, using economic limits will appropriately scale the plot point fluctuations relative to the target Cp value.
    3. A Target IX-MR chart because there’s only one gauge block used in the study and the results are best analyzed as deviation from the known reference value.
    4. A Traditional 3-D chart, because there are actually three components of variation: study-to-study, within-study and over time. In addition, the MR yields the appropriate centerline to be used to calculate the Xbar chart control limits.
    5. A Group 3-D chart, because the three sources of variation should be grouped by associated reference values.

The correct answer will appear in the next issue of InsideSixSigma.


About The Author

Steve Wise’s picture

Steve Wise

Steve Wise is the vice president of statistical methods for InfinityQS, helping companies from all industries implement real-time production for statistical process control and advanced statistical tools. He co-authored  an  industry standard, “D1-9000 Advanced Quality System” in 1991 for Boeing suppliers. Wise is co-author of the book Innovative Control Charting: Practical SPC Solutions for Today's Manufacturing Environment (ASQ Quality Press, 1997).