About this lesson
In this lesson the strategy for how to use Control Charts is discussed. In addition to monitoring the process, the charts can be used to set performance baselines, validate the impact of improvement, and identify sources of variation.
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Statistical Control Stategies
SPC Control Charts can be used for more than just maintaining control of an existing process. The charts can be used to set performance baselines, validate the impact of improvement, and identify sources of variation.
When to use
Statistical control strategies, especially those involving control charts, can be used throughout the improvement process in addition to maintain control during operations. In particular, on a Lean Six Sigma project they can be used during the Measure phase to establish a performance baseline, during the Analyze phase to identify special causes and common causes for variation, during the Improve phase to validate the impact of a performance improvement, and during the Control phase to ensure the change is fully implemented and becomes the new standard process.
Statistical process control can be used both during normal operations and during the improvement process. The strategies in each case changes slightly. During operation the strategy is to reach an ideal state of process performance. During problem solving the strategy is to improve performance and regain stable operations.
Normal Process Operations
During normal process operations a process can be in one of four states depending upon whether or not it is in statistical control and whether or not it is consistently providing results that conform with the process specifications or requirements. This is shown in the diagram below.
When the process is in statistical control and producing conforming results, an “Ideal” state exists. The process will consistently deliver conforming results.
When a process is in statistical control but not always delivering conforming results, it is in a “Threshold” state. Because of the statistical control it is predictable, but that predictability includes a cost of quality for those instances when the results are not conforming. The process manager is on a threshold. He or she must decide whether the cost of improvement to reduce common cause variation is greater than the cost of non-conformance.
When the process is not in statistical control and not producing conforming results, we say it is in a state of “Chaos.” Fortunately, this is normally very obvious and therefore it is relatively easy to initiate an improvement project. The SPC charts provide insight into how bad things are.
The final quadrant is the most dangerous. In this case the process is not in statistical control, but luckily it has not yet created any non-conforming results. We call this state the “False Sense of Security.” Since it is not in statistical control, it is not predictable. The result tomorrow could be non-conforming. However, unlike the Chaos and Threshold states, we don’t expect non-conformances and often are not checking for them. That is why an SPC chart in this case is a crucial element of process management. It will alert you to problems.
Process Improvement Projects
SPC charts and statistical control can also be used very effectively during an improvement project. I will illustrate this by discussing how to use SPC control charts in the phases of a Lean Six Sigma project.
Define – Control charts may occasionally be used in this phase to help justify the need for a project. However, this is not common.
Measure – Control charts are used in this phase to measure and record process performance. During this phase, the process is measured and mapped. Control charts are an excellent tool for use in this phase.
Analyze – Control charts are used in this phase to assist in the identification of special cause variation and an assessment of the level of common cause variation. Control charts do this very well since they were designed to identify special cause variation.
Improve – Control charts are often used in this phase to demonstrate the impact of a corrective or preventive action. A “before” chart is created that illustrates the process performance including mean and control limits before the change is made. Then after the change is made, an “after” chart is created with the new mean and control limits. Generally, we seek to center process mean and narrow the band between the control limits. These charts show whether that improvement has been successful.
Control – Control charts are a primary tool for this phase. Once the improvement is in place, control charts will notify the operator and process manager of problems maintaining the improved performance.
Hints & tips
- Control charts are easy to create and use, so use them. They are a big help in both managing a process and improving it.
- It is often hard to get approval to improve a process that is the “False Sense of Security” quadrant. Since it is not currently a problem an improvement is deferred; the business wants to focus on current problems. The use of a control chart will at least notify the process operators and process management when things suddenly get worse so they can at that time take action.
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