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 is 4: Traditional 3-D chart.

These charts are also known as 3-Way charts. A 3-D chart is a display of three time-ordered lines on three different types of charts. The top chart is the average (Xbar) of the subgroup. In this case, a subgroup represents five readings of the same gauge block. The plot point patterns on this chart will illustrate changes in the mean (bias) over time. The second chart is the moving range (MR) of the top chart. This chart will provide a graphical representation of the measurement system’s stability. The centerline of this chart is used to calculate the control limits on the top chart. The third chart is called the Range-Within (Rw) chart. This is where one plots the subgroup ranges. The Rw chart tracks the measurement system’s repeatability in a time-ordered fashion. Rising plot points typically indicate a problem with the appraisers’ ability to consistently repeat the same measurement. These charts are very sensitive with detecting changes in a measurement system. One could argue that this control chart method is a better approach to signal when a gauge needs maintenance.