A control chart, sometimes referred to as a process behavior chart by the Dr. Donald Wheeler, or Shewhart Charts by some practitioners named after Walter Shewhart. The control chart is meant to separate common cause variation from assignable-cause variation.
Use: A control chart is useful in knowing when to act, and when to leave the process alone. One of the things shown in the course is that if we adjust the process when it really should be left alone, we are actually increasing variation, instead of decreasing it. And, the most conscientious worker might be continually adjusting the process to the target not realizing that they are actually broadening the process variation.
The only time that an operator should make an adjustment to the process is if the behavior of the variation changes significantly. How do you know whether or not the variation has change significantly, that’s the role of the control limit. But it’s not just the control limits that will tell you whether or not that the variation in the process is not behaving itself. There are rules that were formulated in the early 1900s by Dr Shewhart the describe patterns, and zones, and give additional rules for when to act when there is assignable-cause variation. The reading assignments list some of the Shewhart Rules for trends and shifts and non-random patterns.
