Laney p' Chart | p Prime Control Chart in Excel

Create an p' Chart by running QI Macros:

  1. Select your data.
  2. Click on QI Macros menu > Control Charts (SPC) > Attribute> p' .
  3. QI Macros will do the math and draw the chart for you.
  4. QI Macros will also turn any unstable points or conditions red.

Go Deeper: p Chart evaluates process stability when counting the fraction defective (or in many healthcare applications, the fraction meeting requirements). The p Chart works for most data sets. However, if your data meets certain criteria you may need to consider an XmR Chart or a Laney p' Chart (p Prime).

Consider an XmR Chart if the denominator does not vary widely from sample to sample and your p chart looks like the one below where the control limits seem too narrow for the data and most points are highlighted as unstable.

p chart with narrow control limits

Here is an XmR Chart run on the same data:

xmr chart to coimpare with p chart and laney p chart

Consider a Laney p' Chart (p Prime) if: the sample sizes in your denominator are large and vary greatly.

A Laney p' Chart will have wider control limits than the p Chart and it will have varying control limits that may help identify unstable points not identified by the XmR Chart. Here is a Laney p' Chart of the same data.

laney p chart

QI Macros add-in for Excel contains a Laney p' Chart template. Click on QI Macros > Control Chart Templates > Attribute (c,np,p,u,g,t) > p Chart to open up the p Chart template. Then click on the p' Chart Skyline tab at the bottom to open up the Laney p' Chart (p Prime) template.


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 p Chart and p' Chart:

  • Specifically offered in the p Chart macro, you can toggle between a p Chart and a p' Chart by inputting either "False" (p Chart) or "True" (p' Chart):

u-prime-toggle


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