About this lesson
The P chart is closely related to the NP Chart. It also tracks units but tracks the percentage of defective units. This lesson explains how the data is recorded and interpreted on the chart. The lesson describes how to create this control chart in both Microsoft Excel and using Minitab. The lesson will include practice creating the chart.
The P chart (plots percentage of defectives) is the attribute data control chart to be used when the focus is on the percentage of good or bad units in subgroups of variable size. It can be easily created in either Microsoft Excel or Minitab.
When to use
Use the P Chart when seeking to control a percentage or rate of defectives from a process with subgroups that vary in size with each instance of counting. It is frequently used to track error rate data.
The P Control Chart tracks the percentage of units in subgroups that are varying in size. A unit in the subgroup could have no defects, one defect, or many defects. The value is the percentage or proportion of the units with at least one defect. P Charts are often used to track error rates in processes.
P Charts can be created in Microsoft Excel or in Minitab. Within Minitab, control charts are created by using the “Stat” pull down menu, then selecting “Control Charts.” Within the Control Charts window, select “Attribute Charts” and then finally select “P.” In the Minitab P Chart panel, you will need to select the data column with your data and a second column that has the number of units in each subgroup.
If creating the P Chart in Excel:
- Count the number of units in each subgroup. The subgroups should have been sized so that:
Where n is the number of units in the subgroup and pbar is the mean of the subgroup proportions. This means that the minimum subgroup size is 10.
- Count the number of defective units within each subgroup.
- Calculate the percentage of proportion of defective units for each subgroup. Then calculate the proportional mean and subgroup size mean.
- Calculate the Upper Control Limit and Lower Control Limit. The UCL and LCL will change with each data point because the number of units (n) is changing with each data point.
the UCL cannot be greater than 1.
the LCL cannot be less than 0.
- Plot the data points, the Mean and the control limits.
- Take appropriate actions to remove special causes or to center your data within the customer spec limits.
Hints & tips
- Control limits will constantly change, but should remain near the same level.
- The LCL can never be less than zero. If the calculation is a negative number, just use zero for your value.
- When plotting the chart in Excel, use the “Line Graph” charting option with lines that overlay, not ones that stack.
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