What are some of the best practices for creating and interpreting control charts?

2 Collect and plot data

Once you have chosen the type of chart, you need to collect and plot the data on the chart. You should collect enough data to represent the normal variation of the process, and plot the data in the order that they occur. You should also label the x-axis with the time or sequence of the data, and the y-axis with the measurement or attribute of the process. Plotting the data correctly will help you see the trends and patterns of the process over time.

3 Set control limits

Control limits are the horizontal lines that indicate the expected range of variation of the process. They are usually calculated based on the standard deviation or other statistical methods, and are not the same as specification limits or customer requirements. You should set the control limits based on the data that reflect the current state of the process, and update them only when there is a significant change in the process. Setting the control limits appropriately will help you distinguish between common and special causes of variation.

4 Analyze signals

Signals are the indications that the process is out of control or unstable. They are usually detected by applying some rules or tests to the data points on the chart. For example, a signal can be a point that is beyond the control limits, or a run of points that are all above or below the center line. You should analyze the signals to identify the possible causes of variation, and take corrective or preventive actions to eliminate or reduce them. Analyzing the signals will help you improve the process performance and quality.

5 Review and update

Control charts are not static, but dynamic tools that require regular review and update. You should review the control charts periodically to monitor the process behavior, and verify the impact of any improvement actions. You should also update the control charts with new data, and adjust the control limits if necessary. Reviewing and updating the control charts will help you sustain the process improvement and maintain the process stability.