p' Control Chart in Excel

The purpose of a p' chart is to evaluate 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 p prime chart.

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 or a modified p chart in Excel

Here is an XmR chart run on the same data.

xmr chart run on p chart data ratio

Consider a p prime chart if: the sample sizes in your denominator vary greatly.

A p prime 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 p prime chart of the same data.

p prime chart in Excel created with QI Macros software

QI Macros contains a p prime chart template. Click on QI Macros, SPC Charts, p chart to open up the p chart template. Then click on the P' Chart Skyline tab at the bottom to open up the p prime chart template.

p prime Laney chart in Excel in the QI Macros software

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To create a P Prime Control Chart using QI Macros...
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