How to Use Control Charts to Keep Six Sigma Improvements on Track
If you apply control charting as a part of your Six Sigma process control plan, you can use the control chart itself to trigger action or to leave things as they are based on what the control chart tells you. Sample data, also called subgroup data, is collected from the process characteristic in which you’re interested. The process must be allowed to operate normally while you’re taking a sample.
If you give the process you’re charting out-of-the-ordinary care or any special treatment, the information from the control chart is invalid. You must allow the process to act as it normally does while you’re creating a control chart for it.
How often you sample depends on how sensitive you want your chart to be to detect trends or other special-cause patterns in the process behavior. At first, err on the side of taking samples very often, and then, if the process demonstrates that it’s stable and in control, you can take samples less often.
A process should be left as it is if it’s stable and predictable (in control) and capable of meeting customer requirements. If special cause variation occurs, however, you must investigate what caused this extraordinary variation and find a way to prevent it from happening again. Some form of action is always required to make a correction and to prevent future occurrences.
After you have collected a minimum of 25 subgroups of data (with two to five measurements in each subgroup), you can calculate the statistics and control limits by hand or by using a software tool such as Minitab or JMP. If you already have historical data, including this data in the analysis is useful to form a strong baseline of information.
Never confuse control chart limits with specification limits! Specification limits represent the voice of the customer. Control charts, however, represent only the voice of the process, something totally different. Discovering how the process performs naturally, apart from whatever its specifications may be, is the purpose of control charts.
Another way of stating this distinction is that control charts determine only whether the process is stable and predictable. They don’t tell you whether the process is capable of meeting customer requirements. Always resist the temptation to interpret control chart limits as specifications and avoid overlaying specification limits onto your control charts.