Technical Mathematics, Sixth Edition

21.7. Process Control

21.7.1. Statistical Process Control

Statistical process control, or SPC, is perhaps the most important application of statistics for technology students. To cover the subject of statistics usually requires an entire textbook, and SPC is often an entire chapter in such a book or can take an entire textbook by itself. Obviously we can only scratch the surface here. Process control involves continuous testing of samples from a production line. Any manufactured item will have chance variations in weight, dimensions, color, and so forth. As long as the variations are due only to chance, we say that the process is in control.

Example 66: If the diameters of steel balls on a certain production line have harmless chance variations between 1.995 mm and 2.005 mm., the production process is said to be in control.

However, as soon as there is variation due to causes other than chance, we say that the process is no longer in control.

Example 67: During a certain week, some of the steel balls from the production line of Example 66 were found to have diameters over 2.005. The process was then out of control.

One problem of goal control is to detect when a process is out of control so that the cause may be eliminated.

Example 68: The period during which the process was out of control in Example 67 was found to occur when the factory air conditioning was out of operation during a heat wave. Now the operators are instructed to stop the production line during similar ...