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.

compare u chart to laney u chart

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

xmr chart compare to laney u chart

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.

laney u chart

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.

u-chart-template-tabs