How do you use SPC to maintain process stability?

1 What is SPC?

SPC is based on the idea that every process has some inherent variation, which can be measured and classified into two types: common cause and special cause. Common cause variation is the normal and expected fluctuation of the process, while special cause variation is the abnormal and unpredictable deviation from the process mean. SPC uses control charts to plot data from a process over time and compare it with control limits, which are calculated from the historical data. The control limits represent the range of variation that is acceptable for the process. If the data points fall within the control limits, the process is stable and in control. If the data points fall outside the control limits, or show a non-random pattern, the process is unstable and out of control, indicating the presence of a special cause.

2 How to create a control chart?

To create a control chart, you need to collect data from your process, preferably in a rational subgroup, which is a set of measurements that are taken under similar conditions. The data should be representative of the normal operation of the process and cover enough variation. You also need to select the type of control chart that is appropriate for your data and your objective. There are different types of control charts, such as X-bar and R charts for variables data, and p, np, c, and u charts for attributes data. Each type of control chart has a center line, which is the average or target value of the process, and upper and lower control limits, which are usually set at three standard deviations from the center line. You can use software or formulas to calculate the center line and the control limits. Then, you plot the data points on the control chart and analyze the results.

3 How to interpret a control chart?

Interpreting a control chart requires looking for signals that suggest the process is out of control. To do this, you can use the Western Electric rules or the Nelson rules. Common signals to look out for include a data point outside the control limits, two out of three consecutive points near the control limits, seven or more consecutive points on one side of the center line, six or more consecutive points increasing or decreasing, fourteen or more consecutive points alternating up and down, and eight or more consecutive points with none in the center third of the control limits. If any of these signals are present on your chart, you should investigate possible causes and take corrective action. After that, you should update your chart with new data and ensure that the process is back in control.

4 How to improve a control chart?

If your control chart shows that your process is in control, but you are not satisfied with the level of variation or the performance of the process, SPC can help improve it. To reduce common cause variation, optimize process parameters, standardize procedures, train operators, and maintain equipment. This will result in a narrower control limit and a lower standard deviation, thus improving capability and quality. Alternatively, shift the center line by implementing a process improvement project such as Lean Six Sigma. This will result in a new center line and new control limits that reflect improved performance.

5 How to maintain a control chart?

Once you have created, interpreted, and improved your control chart, you need to maintain it to ensure that your process remains stable and in control. This includes collecting and plotting data regularly and consistently, reviewing and analyzing the control chart periodically for any signs of instability, investigating and eliminating any special causes of variation, updating the control chart with new data and recalculating the center line and control limits if necessary, communicating and documenting the results and actions taken, and using the control chart as a basis for continuous improvement. SPC is a powerful tool that can help you maintain process stability and achieve your quality goals. It is not a one-time activity but rather a continuous process that requires attention and commitment. By following these steps, you can use SPC to maintain process stability and deliver value to your customers.

6 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?