Control charts are graphs that study how a process will change over time. Control charts always have a central line for the average, and control limits that are represented by lines for both the upper control limit and the lower control limit.
The control limits are lines on a control chart used as a basis for judging whether variation in data on a chart is due to special or common causes. They are important in helping your organization react to statistically significant signals in the data and avoid over controlling or under controlling. Control limits are calculated in such a way as to anticipate the common cause variation in a system. They are calculated on the basis of approximately three standard deviations from the average of the data.
Control charts are often used in pairs. The top chart monitors an average of distribution data, while the bottom chart monitors range and width of distribution. They are highly effective tools to study how a process or system changes over time. Control charts discriminate special from common causes of variation, to help you understand how to improve a process. Control charts are useful for understanding all types of data collected over time and help you take timely action when problems occur.
Control charts should be used in your business to:
- Monitor ongoing processes to find and correct problems as they occur
- Predict the expected range of outcomes
- Determine if a process is stable
- Avoid reacting when you shouldn’t (over controlling) or failing to react when you should (under controlling)
- Determine if process variation is due to special causes or common causes
To successfully use control chart visuals for improvement, follow these steps:
- Select data collected over time
- Select the appropriate control chart for the type of data you have collected
- View the signals on the chart to ascertain the type of variation in the system
- Investigate any special causes of variation
- If the system is stable, or in control, work on the causes of ordinary events in order to reduce variation
- If the system is unstable, or out of control, work to eliminate special causes of variation
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