About this lesson
Control Chart Design
The control chart is carefully designed to provide a visual representation of process stability.
When to use
When creating a control chart, the design methodology must be used so that the data is correct. And of course, when using control charts, the design of the control chart must be understood so that proper conclusions are drawn concerning process stability and process control.
Control charts are carefully designed so that they reveal when a process is stable or unstable. They show the magnitude of the common cause variation which is always occurring in stable processes. They also indicate the introduction of special cause variation, which leads to process instability.
The figure below shows a control chart. The horizonal axis is time or the sequence of data points. This gives the chart a time-wise view of the process. The vertical axis is the magnitude of the data being charted. The mean or average value is plotted, which provides a sense of expected value. There are also two horizontal control limit lines plotted, which indicate the magnitude boundaries of the normal or common cause variation. These lines are called the upper control limit and the lower control limit. The plot of the data will reveal the introduction of special cause variation. This can be seen whenever data falls outside the control limits and in some cases when certain patterns develop in the data, even though it is inside the limits. These patterns will be discussed in other modules.
Hints & tips
- The method for calculating control limits varies depending upon the type of data and the chart. These will be discussed in detail in other modules.
- Statistical software can be programmed to automatically show when special causes exist. However, the rules are simply to apply so charts can be created with a simple software application like Excel and still be used to identify special cause variation.
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