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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.

Exercise files

Download this lesson’s related exercise files.

NP Chart.xlsx
10.1 KB NP Chart - Solution.docx
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Quick reference

NP 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.

Instructions

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:

  1. 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.

  1. Count the number of defective units within each subgroup.
  2. Calculate the Mean and the Upper Control Limit and Lower Control Limit. 
    1. The control chart formulas label the count of defectives as np.

  3. Plot the data points, the Mean and the control limits.
  4. 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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  • 00:04 Hi, I'm Ray Sheen.
  • 00:05 It's now time to take a look at the attribute data control chart
  • 00:09 know as the NP chart.
  • 00:13 The NP chart shows the number of defective units in each subgroup.
  • 00:17 The data being charted is the attribute data of pass/fail or yes/no.
  • 00:23 The subgroup for
  • 00:24 the NP are all fixed in the same time but this will simplify some things for us.
  • 00:29 But in this case,
  • 00:30 what is being tracked is the pass/fail of a unit not just an attribute.
  • 00:35 So it's not the number of defects, it is the number of defectives that matters.
  • 00:40 And since in the NP chart the subgroup size is fixed
  • 00:44 we can use the actual count of defective units and not need to normalize it.
  • 00:49 This is a great chart for a quick high level look at how many good and
  • 00:53 bad units are flowing to customers.
  • 00:56 It doesn't get lost in the details, but
  • 00:58 just checks if a system is 100% good or not.
  • 01:02 The NP chart is easy to understand and easy to create.
  • 01:06 And since it is counting the unit level it's easy to explain the effect
  • 01:10 of the numbers.
  • 01:12 Let's look at an example of a NP Chart.
  • 01:14 Of course, it has the standard control chart elements of mean and control limits.
  • 01:18 As you can see, the y-axis is the number of counts of defective units.
  • 01:24 So we need to count the number of defective units in each subgroup and
  • 01:28 then plot that value.
  • 01:30 Like we have come up before with other attribute charts the lower control limit
  • 01:34 cannot be less than zero.
  • 01:37 Now we went through the steps of creating a control chart in a previous module, but
  • 01:41 let's look at some specific items about the NP chart.
  • 01:45 So we'll start with subgroup size.
  • 01:47 This is usually a calendar time such as a day or a week.
  • 01:51 The subgroup should be large enough, the percentage or
  • 01:53 proportion times the number of units is greater than 5.
  • 01:57 If the percentage is greater than 50%, we change that formula so it's 1
  • 02:02 minus the proportion times the number of units needs to be greater than 5.
  • 02:07 Once your subgroup size is set,
  • 02:09 count the number of defective units in each subgroup.
  • 02:13 Again, we're talking about defective units and not the total number of defects.
  • 02:18 When you have enough data points, calculate your mean and control limits.
  • 02:22 Once these limits are set you probably don't need to recalculate them with each
  • 02:26 new data point but only when a process change has occurred.
  • 02:30 Now plot your data points, the mean, and control limits.
  • 02:33 And of course if the plot shows that the chart is not in control,
  • 02:37 take action to stabilize it.
  • 02:39 Let's look at how we do the calculations manually or in Excel.
  • 02:43 And as with other charts, formulas are on the right side of the screen and
  • 02:46 the calculation steps will be discussed on the left.
  • 02:49 The first step is easy,
  • 02:50 count the number of defective units, put that in your spreadsheet.
  • 02:54 I normally put that data in a column.
  • 02:57 This count is called the np value.
  • 03:00 Divide this by the number of units in the subgroup
  • 03:02 to determine the proportion value of defectives for that subgroup item.
  • 03:06 This is the p value, and I will put that in the next column.
  • 03:11 Once you have enough data points, and we recommend 30, you can calculate a mean or
  • 03:15 average of the np value for that dataset and an average for the p value.
  • 03:20 Now calculate the control limits.
  • 03:23 This is done by multiplying three times, the square root of the np
  • 03:28 mean value, times 1 minus the average of the p value.
  • 03:33 Then add that product to the np mean value for the upper control limit.
  • 03:38 And subtract the product from the np mean value for the lower control limit.
  • 03:43 But remember the lower control limit cannot be less than 0.
  • 03:48 If you're using Excel, you can plot your data, the mean value, and the upper and
  • 03:52 lower control limits using the Line Chart graphing option.
  • 03:57 Now let's look at creating this chart in Minitab.
  • 03:59 Go to the Stat menu, select Control Charts, select Attribute Charts and
  • 04:04 finally select the NP Chart.
  • 04:06 When you do that you should get a panel that looks like this.
  • 04:09 Place your cursor in the variables window and click to activate the column display.
  • 04:14 Then highlight the column where you're data is located and
  • 04:17 click the Select button.
  • 04:19 You're data column should now be in the variables window.
  • 04:23 With the NP chart, you must set the Subgroup size also.
  • 04:27 Since it is a fixed size, I enter the value of units for
  • 04:30 each subgroup in the Subgroup size window.
  • 04:34 Now, click on the OK button at the bottom of the panel, and
  • 04:37 Minitab will generate your control chart.
  • 04:40 So, that's the NP Chart, the control chart for
  • 04:43 tracking the number of defective units that you're working with.

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