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A np chart is used to examine the variation in the number of defective items in a group of items. The subgroup size is constant with an np chart. An item is defective if it fails to conform to some preset specification. The np control chart is used with “yes/no” attributes data. This means that there are only two possible outcomes: either the item is defective or it is not defective. For example: either the phone is answered or it is not answered. For more information on np control charts, please see our SPC Knowledge Base.

This page shows you how to make the np chart. The data can be downloaded at this link. This page contains the following:

Data Entry

A company uses telemarketing to help boost sales. The sales manager is interested in tracking the % of telemarketing calls that result in an order each day. The data he has collected are given below. He wants to use that data to construct a np chart. “np” is the number of “defective” items — in this case, the number of calls that result in an order. The values of np are plotted over time. The sales manager randomly selected 50 calls per day.

Enter the data into a spreadsheet as shown below. The data does not have to start in cell A1. It can be anywhere on the worksheet. The data can be in columns or rows.

Creating a New np Chart

This is all that is needed to make the chart. If you select OK at this point, the software will generate the control chart using the default options. Simple and quick.

The options on the input screen are:

Updating the np Chart with New Data

The np chart can be easily updated with new data after it has been added to the spreadsheet. Please see Updating Control Charts with New Data to see how the software finds the new data and updates the chart.

Changing the Options for the np Chart

You can change the current options for a np chart (e.g., changing the out of control tests) by selecting “Options” on the Updating/Options panel on the SPC for Excel Ribbon. See Changing Chart Options for more information

In addition, once a control chart is made, there are numerous actions you can take on the chart including splitting control limits, removing points from the calculations, adding comments, selecting the range on which to base control limits, etc. Please see Control Chart Actions for details.

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