All processes are affected by various sources of variations over time. Products that are designed based on optimal settings will, in reality, tend to drift away from their ideal settings during the manufacturing process.

Environmental fluctuations and process variability often cause major quality problems.

Focusing only on costs and performances is not enough. Sensitivity to deterioration and process imperfections is an important issue. It is often not possible to completely eliminate variations due to uncontrollable factors such as temperature changes, contamination, humidity, and dust.

For example, the products sold by your company might go on to be used in many different environments across the world, or in many different (and unexpected) ways, and your process, even when it has been carefully designed and fine-tuned, will tend to vary over time.

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