Laney u chart | u' prime charts in Excel
The purpose of a u chart is to determine stability of "counted" data (e.g., errors per bill, dents in a car door, etc.) when there can be more than one defect per unit and the sample size varies.
The u chart works for most data sets. However, if your data meets certain criteria you may need to consider an XmR chart or a u prime chart.
Consider an XmR chart if the denominator does not vary widely from sample to sample and your u chart looks like the one below where the control limits seem too narrow for the data and most points are highlighted as unstable.
Here is an XmR Individuals chart run on the same data:
Consider a Laney u prime chart if: the sample sizes in your denominator vary greatly.
A Laney u prime chart will have wider control limits than the u chart and it will have varying control limits that may help identify unstable points not identified by the XmR chart. Here is a Laney u chart of the same data.
QI Macros add-in for Excel contains a Laney u' chart template. Click on QI Macros, SPC Charts, u chart to open up the u chart template. Then click on the U' Chart Skyline tab at the bottom to open up the Laney u prime chart template.
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