About this lesson
The NP Chart tracks defective units rather than actual defects. It provides a more macro perspective for the organization. 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 NP chart (plots number of Defective units) is the attribute data control chart to be used when the focus is on the number of failed or defective units and the subgroup size is fixed. It can be easily created in either Microsoft Excel or Minitab.
When to use
Use the NP Chart when counting the number of defective units within a fixed subgroup size. It is frequently used to track the failure of a process to deliver results, without digging into the details of why it did not deliver.
The NP Control Chart tracks the number of defective units in the subgroup. A unit in the subgroup could have no defects, one defect, or many defects. The value is the number of units, not the number of defects. This is normally a whole number value.
NP 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 “NP.” In the Minitab NP Chart panel, you will need to select the data column with your data and provide a number for the quantity of units in the sample. This should be a number that you enter.
If creating the NP Chart in Excel:
- Establish an operational definition of the subgroup size. This should be a fixed number of units. Subgroups should be sized so that:
Where n is the number of units in the subgroup and pbar is the average percentage of defective units.
- Count the number of defective units within each subgroup.
- Calculate the Mean and the Upper Control Limit and Lower Control Limit.
- The control chart formulas label the count of defectives as np.
- 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
- Set your control limits once you have 30 data points. They do not need to be recalculated unless you change the process or remove a special cause condition.
- 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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