For variables control a circuit voltage will be measured based on a sample of only five circuits. The past average voltage for samples of size 5 has been 3.1 volts, and the range has been 1.2 volts.
Control charts can indicate when the adjustments are necessary and when they are not. To determine the natural range or control limits of a process and to compare it to its specified limits. If the range of the control limits is wider or higher than the one of the specified limits, the production process will need to be accustomed to inform about the process capabilities and stability.
Samples need to be taken on a controlled regular basis and tested to make sure that the quality is acceptable.
Clarkson, Elizabeth. "A Process Control Primer" February 24, 2000 from: http://members.cox.net/bethclarkson/Articles/ControlCharts.html
Bass, Issa. "Control Charts" Sixsigmafirst Group, Inc.2005 from: http://www.sixsigmafirst.com/controlcharts1.htm