The primary SPC tool for Six Sigma is the control chart — a graphical tracking of a process input or an output over time. In the control chart, these tracked measurements are visually compared to decision limits calculated from probabilities of the actual process performance. The visual comparison between the decision limits and the performance data allows you to detect any extraordinary variation in the process.
The control chart you choose is always based first on the type of data you have and then on your control objective. The control chart decision tree aids you in your decision.
The general step-by-step approach for the implementation of a control chart is as follows:
Define what needs to be controlled or monitored.
Determine the measurement system that will supply the data.
Establish the control charts.
Properly collect data.
Make appropriate decisions based on control chart information.
Control charts provide you information about the process measure you’re charting in two ways: the distribution of the process and the trending or change of the process over time. You use control charts to
Provide a simple, common language for discussing the behavior and performance of a process input or output measure
Control the performance of a process by knowing when and when not to take action
Reduce the need for inspection
Understand and predict process capability based on trends and other performance insights
Determine whether changes made to the process are having the desired result
Provide an ongoing, continual view of the performance of the process
Create a repository of data for follow-on improvement activities