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Key SPC Assumptions - A Wayworld Tutorial

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• Variation in a process is normal, natural and can not be eliminated entirely.

At some level of measurement, all processes result in variation.

• Manufacturing is a process.

The point of the process is to create or add value.

• Variation in manfacturing process results = waste.

The higher the variation, the higher the waste.
The higher the waste, the lower the value added per unit of resource used.

• Reduction in variation is "Good".

Reduction in variation reduces waste, reducing cost per unit.
Reduction in variation makes competitive specifications, increasing price per unit.
Reduction in variation allows controlled continious process improvement

• Variation in process is statistical.

A single measurement can not determine variation in process.
Measuring a process over time requires multiple measurements.
Statistical tools are used to describe the variation.

• Variation in process is of two types; "Special" or "System".

"Special" variation is when something odd or outside the system happens, like a single mistake. Once you find source of special variation, the cause can be changed without changing the whole system.
"System" variation is the natural variation produced by the system. To reduce system variation, the system itself must be changed.

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