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:
- The opportunity for defects to occur must be large
- The actual number that occurs must be small
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
- Creating a new c chart
- Updating the c chart with new data
- Changing the options for the c chart
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
1. Select the data on the worksheet to be included in the analysis. You can use “Select Cells” in the “Utilities” panel of the SPC for Excel ribbon to quickly select the cells.
2. Select “Attribute” from the “Control Charts” panel on the SPC for Excel ribbon.
3. Select “c Chart” and then select “OK”.
4. The input screen below for the c chart is displayed. The program sets the initial subgroup identifiers and c values ranges based on the range you selected on the worksheet. This is why there is an advantage to selecting the sample and data ranges prior to making the chart. You can edit the ranges if needed here. Enter a name for the chart. In this example, the name “Returned Goods” is used as the name of the chart. This is the name that will appear on the worksheet tab containing the control chart. It must be unique – there cannot be another workbook tab with that name.
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:
- Subgroup Identifiers: the worksheet range containing the subgroup identifiers
- c values: the worksheet range containing the number of defects
- Name of Chart: chart name – must be a unique name in the workbook and is limited to 25 characters
- Subgroup Identifiers: this information is used to search for new data when updating the chart (see below)
- In One Column: the subgroup identifiers are in one column (as in this example)
- In One Row: the subgroup identifiers are in one row
- Automatic Updating of Limits: option controls whether the average and control limits are updated as new data are added, default value is Yes; set this to No to stop the average and control limits from updating when new data are updated.
- Show Options: selecting this button shows the various options for this chart
- Out of Control Tests
- Control Limit Options
- Titles and Formats
- Manual Control Limits
- Chart Location
- Select OK to create the chart
- Select Cancel to exit the software
5. Once you have select the options you want (again, no options are required to make the chart), select OK and the chart will be generated. An example based on the data above is shown below.
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.