This chart is primarily used to control THE PRODUCTION LINE of MANUFACTURED PRODUCTS. For example, ballbearings.

Quality control engineers take RANDOM SAMPLES of ball bearings from a production line and inspect each by a go-nogo gauge. The number of rejects are plotted on a QUALITY CONTROL CHARTS, such as that above.

The CHART has UPPER and LOWER CONTOL LINES to bound out EXTREME VALUES. If a REJECT NUMBER falls above the UPPER CONTROL LINE, this is a warning to the engineers that THE PROCESS MAY BE GOING OUT OF CONTROL, and could become worse. They look into methods for bringing the PROCESS back UNDER CONTROL, so that most of the ball bearings will PASS INSPECTION. If a REJECT NUMBER falls below the LOWER CONTROL LINE, the engineers realize that "something good may be happening" -- better supplies, better workmanship, or whatever. They try to find a "cause" and take advantage so that THE QUALITY AVERAGE INCREASES.

This is "engineering-by-exception". The engineers need only bother with the exceptional cases that "fall outisde" the control-limits -- a great saving in time, money, and other resources!

By similar methods for various types of equipment, QUALITY CONTROL helped to win WORLD WAR II and enriched the economy and improved out lives in peace-time.