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About this lesson
The C chart (plots Counts) is the simplest of the attribute data control charts. 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 C Chart (plots Counts) is the simplest of the attribute data control chart. It can be easily created in either Microsoft Excel or Minitab.
When to use
Use the C Chart when it counts the number of an attribute occurrence (defect) within all the units of a fixed subgroup size. It is frequently used to count the occurrences of an attribute during a fixed time period such as a day or week.
The C Control Chart tracks the count of occurrences of an attribute (such as a defect). The value is always a whole number. The count is for every occurrence of that attribute within a sample or subgroup of a fixed size. C Charts are often used to count the number of occurrences during a fixed time period such as a day or week.
C 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 “C.” In the Minitab C Chart panel, all that needs to be done is to select the data column with your data.
If creating the C Chart in Excel:
- Determine your subgroup size. The subgroup size must be constant for the C chart. The subgroup size should be such that the average or mean value of the counts is greater than two.
- Count the occurrences of the attribute or defect (not the number of defective units) within each subgroup.
- Calculate the Mean and the Upper Control Limit and Lower Control Limit
- 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. In the example shown there is a special cause condition of a mean shift indicated at point 18 that needs to be investigated.
Hints & tips
- This is the easiest of the attribute data control charts to understand and create.
- Set your control limits once you have 30 data points. They do not need to be re-calculated 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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