A Control Chart is also known as the Shewhart chart since it was introduced by Walter A Shewhart. We can also call it as process behavior chart. By this, we can see how is the process behaving over the period of time. Although in Six Sigma study, we usually read Control chart in the Control phase. However, a control chart is being used at the initial stage to see the process behavior or to see the Voice of Process (VoP). If our process is stable its’s only then we should think about running the project. Else, at first, we should make the process stable. It is one of the seven effective quality tools.
If we say the process is stable, which means that all the data points fall under the control limits, no special reason which is making process unstable.
As a matter of fact, we have variations everywhere, no process is without variation. This means that there can be no common cause variation or special cause variation. In the control charts, we see how these variations impact our process over a period of time, whether our process will be in control or will cross the process boundaries. Control charts help us in visualizing this variation. Control charts have one central line or mean line (average), and then we have the Upper Control Limit (UCL) and Lower Control Limit (LCL). The upper control limit and lower control limit are three standard deviation distance from the center line in both sides. We can have the upper warning line and lower warning limit also. Now the question is which is the two standard deviation in distance from the central line? The one which alarms us if data points crossing this limit, this can make the process unstable.
We use a control chart to see the special cause variation. Special cause variation does not always indicate the negative part of the process, sometimes it reflects a good indication for the process too. If we have some special cause due to that we have process variation, we can adopt the preventive actions to avoid those special cause variation in the future. Likewise, if we get late due to a flat tire, we could take some preventive actions to avoid such situations in the future. The special cause is also called as the assignable cause as it is avoidable, while the common cause is inevitable.
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One of the reasons to use a control chart is to see whether our process is stable or not if we find the process as unstable, we need to work on this. It even gives the discern between the assignable or unassignable causes for variations. The control chart tends to make a process simple while skipping the assignable causes.
It helps to detect the process average, and estimate the variation (the spread in the histogram). We need to understand that the process in control is more important. Also, you need to check the process mean, and all the data points should fall between the Upper and Lower Control Limits. By doing this, we can judge whether our process is capable enough or not and also what we want to do with our process.
Where can we check the Process Capability by Cp and Cpk?
While using the control chart, we could see the process improvement, while seeing the process average and we can compare it with the earlier process mean. This gives us the information about how much our process is in control. Like as a normal chart here in control chart, we have the same rules, 68% of data points should fall under the 1st standard deviation, and 95% data points should be within the 2nd standard deviation, and 99.7 % data should be within 3rd standard deviation.
Apart from these, there are other points too. They are:
· When do we need to predict the outcomes range?
· How to check the stability?
· When do we need to see the pattern of process variations?
· When & How does quality improvement help in understanding the specific problem and preventing them?