Laney p Chart, p' Prime 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 Laney or 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 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 or p prime chart if: the sample sizes in your denominator are large and 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.

laney p chart

 

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