How to Use a Control Chart

Use a control chart to tell you whether your process is out of control, unstable, or not predictable. A control chart tells you how much variation the process causes. A stable process produces predictable results consistently. Processes that are "out of control" need to be investigated and stabilized before they can be improved.

use a Control Chart  to determine process stability

Causes of Variation

There are two different causes of variation. Common causes of variation happen all the time, every day. Getting from your home to school or work takes varying amounts of time because of traffic or transportation delays.

Special causes, require immediate cause-effect analysis to eliminate the special cause of variation. A blizzard, a traffic accident, a chemical spill or other freak occurrence would be a special cause of variation.

Any point outside the upper or lower control limits is a clear example of a special cause. The other forms of special cause variation are called "runs." Trends, cycling up and down, or "hugging" the center line or limits are special forms of a run. Use a control chart to identify and investigate these special causes.

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