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 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 u chart looks like the one below where the control limits seem too narrow for the data and most points are highlighted as unstable.

u chart vs modified u prime chart

Here is an XmR Individuals chart run on the same data.

Xmr chart in Excel using modified u chart 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.

u prime chart in Excel created with QI Macros software

QI Macros contain a u prime 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 u prime chart template.

modified u prime chart in QI Macros software

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To create an U Prime Control Chart using QI Macros...
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