The question “Is this batch like the others?” is asked all over the world on a daily basis. It turns out that the process behavior chart provides a very effective answer for questions about the homogeneity of the product stream.

In World War II, Gen. Leslie Simon used the process behavior chart to define “grand lots” of material. Material in the same grand lot should all be treated alike, but material belonging to different grand lots might need to be treated differently.

An example of this concept comes from an automotive supplier making a hydraulic brake hose. This hose consists of a rubber tube, surrounded by a reinforced braided tube of synthetic fiber, and covered by an outer layer of rubber. The government specification for the minimum tensile strength for the synthetic fiber is 5.0 pounds. The automobile company required a minimum tensile standard of 5.5 pounds. The OEM supplier had a more conservative internal minimum standard tensile of 6.0 pounds. (A burst brake hose can have serious consequences.)

 …