The idea behind statistical process control (SPC) is that over a period of time you can take measurements on a process and determine whether that process is going out of control. Examples of process measurements include
Number of defective units in a manufacturing run
Average number of defects in a unit
Average diameter of a part that must meet a precise specification
Average number of errors in an invoice
Average length of time that customers remain on hold
Average available capacity of a data communications circuit
or virtually any other process that your company uses.
The key phrases here are “over time,” “average,” and “out of control.” Over time means that SPC depends on repeatedly and regularly ...