1. Proven technique for improving productivity.
2. Effective in defect prevention.
3. Prevention of unnecessary process adjustments.
4. Providing diagnostic information.
5. Providing information about process capability.
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.
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.