When is it used?
A c-chart is particularly useful when the item is too complex to be ruled as simply conforming or nonconforming. For example, an automobile could have hundreds of possible defects yet still not be considered defective. A c-chart could also be used to monitor the number of phone calls received in a given period of time such as an hour or a day.
Use c-charts when you can answer "yes" to these questions:
- Do you need to assess system stability?
- Is the data the number of nonconformities per subgroup?
- Are the subgroups the same size?
- Have the possible nonconformities been identified prior to data collection?
- Is the time order of subgroups preserved?
Getting the most
Collect as many subgroups as possible before calculating control limits. With smaller amounts of data, the c-chart may not represent variability of the entire system. The more subgroups you use in control limit calculations, the more reliable the analysis will be. Typically, 20 to 25 subgroups will be used in control limit calculations.
C-charts have several applications. When you begin improving a system, use them to assess the system’s stability .
After the stability has been assessed, determine if you need to stratify the data. You may find entirely different results between shifts, among workers, among different machines, among lots of materials, etc. To see if variability on the c-chart is caused by these factors, collect and enter data in a way that lets you stratify by time, location, symptom, operator, and lots.
You can also use c-charts to analyze the results of process improvements. Here you would consider how the process is running and compare it to how it ran in the past. Are there fewer nonconforming units?
Finally, use c-charts for standardization. This means you should continue collecting and analyzing data throughout the process operation. If you made changes to the system and stopped collecting data, you would have only perception and opinion to tell you whether the changes actually improved the system. Without a control chart, there is no way to know if the process has changed or to identify sources of process variability.