Statistical process control
SPC relies on quantitative and graphic analysis of measurements to evaluate observed variation. If the attributes of interest (content weight in this example) vary within an acceptable range, a process is said to be in control, in statistical control, or stable. When unacceptable variation is noted, actions are typically taken to determine and correct their cause. In the bottling example, suppose too many bottles are filled with less than 245 grams. Checking the plant equipment reveals that one of ten filler valves is malfunctioning.