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A c control chart is used to monitor the variation in the number of defects. A defect occurs when something does not meet a preset specification. A c control chart is used with counting type attributes data (e.g., 0, 1, 2, 3). These are whole numbers. In addition, to use a c control chart, two other conditions must be true:

With a c control chart, we are often looking at an area, not a group of items. For example, we may use a c control chart to monitor injuries in a chemical plant. In this case, the subgroup is the plant. The opportunity for injuries to occur is large; the actual number that occurs is small relative to the opportunity. With a c control chart, the area of opportunity for defects to occur must be constant. For more information on c control charts, please see our SPC Knowledge Base.

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

Data Entry

A distributor is monitoring the number of returned goods per day. The data are shown below. “c” is the number of returned goods each day. For the first date, there were 20 goods returned. 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 c 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 c Chart with New Data

The c 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 c Chart

You can change the current options for a c 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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