special cause

Reduce Special-cause Variation Before Experimentation

For several years, a fully-automated plastic drinking cup production line used excessive amounts of raw materials (plastic PET pellets) due to a wide distribution in the weight of the formed cups. When process operators and engineers had tried to reduce the plastic pellet usage by reducing the average formed cup weight, many cups – because…

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Seeking the Causes of Variation

Not all variation is created equal. Planned variation, like that in an experiment, is a process improvement strategy. Unplanned variation, however, is nearly always bad. Two types of variation concern a Six Sigma team: Common cause variation – All processes have common cause variation. This variation, also known as noise, is a normal part of…


Remember Common Cause Variation in Fight Against Waste

Ideally, practitioners want an error rate to be at 0 percent. But if an error rate is steadily hovering at about 2 percent, it is fair to say that the errors are due to common cause variation. If the error rate suddenly surges beyond its usual level, however, it is probably due to some change…