SPC is the practice of measuring a process while it runs and plotting the results on control charts, so you can tell normal variation from a real change. Control limits are calculated from the process's own data, usually three standard deviations either side of the average. A point outside the limits, or a pattern such as a long run on one side of the center line, signals that something specific has changed and should be found before bad parts are made. Control limits are not specification limits.
Example: An operator measures five parts every hour and plots the average and range. One afternoon, seven averages in a row sit above the center line, still inside tolerance. The setter checks the tool, finds it wearing, and changes it before any part goes out of spec.
SPC moves a plant from sorting bad parts to reacting before they are made. It also stops over-adjustment: if operators tweak a machine every time a reading moves within normal variation, they add variation instead of removing it.
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