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P-Chart: Proportion Control Chart

A free online reference for statistical process control, process capability analysis, measurement systems analysis,
control chart interpretation, and other quality metrics.

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p-chart

What is it?

A p-chart is an attributes control chart used with data collected in subgroups of varying sizes. Because the subgroup size can vary, it shows a proportion on nonconforming items rather than the actual count. P-charts show how the process changes over time. The process attribute (or characteristic) is always described in a yes/no, pass/fail, go/no go form. For example, use a p-chart to plot the proportion of incomplete insurance claim forms received weekly. The subgroup would vary, depending on the total number of claims each week. P-charts are used to determine if the process is stable and predictable, as well as to monitor the effects of process improvement theories. P-charts can be created using software programs like SQCpack.

What does it look like?

The p-chart shows the proportion of nonconforming units in subgroups of varying sizes.

When is it used?

Use a p-chart when you can answer “yes” to all these questions:

1. Do you need to assess system stability?

2. Does the data consist of counts that can be converted to proportions?

3. Are there only two outcomes to any given check?

4. Has the characteristic being charted been operationally defined prior to data collection?

5. Is the time order of subgroups preserved?

Additional reference material

Additional sections from legacy p-chart-f:

p-chart formulas

The p formula (for the proportion of nonconforming units from subgroups that can vary in size):

To calculate control limits for the p-chart:

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