Control charting is one of the tools of Statistical Quality Control (SQC). It is the most technically sophisticated tool of SQC. It was developed in the 1920s by Dr. Walter A.
Stewart of the Bell Telephone Labs, as a statistical approach to the study of manufacturing process variation, for the purpose of improving the economic effectiveness of the process.
These methods are based on continuous monitoring of process variation. Control chart is a device for describing in a precise manner what is meant by statistical control. Its uses are as follows:
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1. Proven technique for improving productivity.
2. Effective in defect prevention.
3. Prevention of unnecessary process adjustments.
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4. Providing diagnostic information.
5. Providing information about process capability.
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A typical control chart is a graphical display of a quality characteristic that has been measured or computed from a sample, versus the sample number or time.
The chart contains a center line that represents the average value of the quality characteristic corresponding to the in-control state. Two other horizontal lines, called the Upper Control Limit (UCL) and the Lower Control Limit (LCL) are also drawn.
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These control limits are chosen so that if the process is in control, nearly all of the sample points will fall between them. As long as the points plot within the control limits, the process is assumed to be in control, and no action is necessary.
However, a point that plots outside the control limits is interpreted as evidence that the process is out of control, and investigation and corrective action is required to find and eliminate the assignable causes responsible for this behavior.
The control points are connected with straight line segments for easy visualization. Even if all the points plot inside the control limits, if they behave in a systematic or nonrandom manner, then this is an indication that the process is out of control.