u' prime charts in Excel

Jay Arthur
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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 uchart 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 uchart 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 u prime chart if: the sample sizes in your denominator vary greatly.

A 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 u prime chart of the same data.

The QI Macros contain a u prime chart template. Click on the QI Macros pull down menu and select Fill-in-the-blank templates, SPC Charts, u chart to open up the u chart template. Click on the U' chart skyline tab to open up the u prime chart template.

• u chart in excel
• p prime chart
• XmR Individuals Chart
• Other types of control charts

The u prime chart is just one of the tools included in the QI Macros for Excel SPC Software for Excel.

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