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