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 Laney u' Chart (u Prime).

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' Chart (u Prime) if: the sample sizes in your denominator vary greatly.

A Laney u' Chart (u Prime) 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 > Control Chart Templates > Attribute (c,np,p,u,g,t) > 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' Chart (u Prime) template.

u-chart-template-tabs

Change the Y-Axis Multiplier:

  • Located to the right of the chart, this functionality allows you to change the multiplier. This automatically changes the y-axis value (e.g. 1, 10, 100, etc.):

y-axis-multiplier

Toggle Between u Chart and u' Chart:

  • Specifically offered in the u Chart macro, you can toggle between a u Chart and a u' Chart by inputting either "False" (u Chart) or "True" (u' Chart). This option is located to the right of your chart:

p-prime-toggle

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