p Chart in Excel
The purpose of a p chart is to evaluate process stability when counting the fraction defective.
The p chart is used when the sample size varies: the total number of circuit boards, meals, or bills delivered varies from one sampling period to the next.
p chart data looks like this:
Example of a p chart created using the QI Macros SPC Software for Excel.
The stair step or skyline control limits are the result of the varying sample sizes.