Separating production & measurement errors

underestimated uncertainty?

underestimated uncertainty?

Underestimation of uncertainty in measurement process

=> Overestimation of product variation

  • Over-adjustment of manufacturing process and worse increase in product variation.

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Overestimated uncertainty?

Overestimated uncertainty?

Overestimation of uncertainty in measurement process

=> Poorer product quality and unnecessary investment in new expensive measurement equipment.

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Different variables entering into conformity assessment of a product

Product variation

  • Variable xspecific: Actual variation in the value of the quality characteristic of one specific item of the product subject to conformity assessment (for example, changes arising when the item is handled in trade)
  • Variable xglobal: Actual variation in the value of the quality characteristic across a population of items of the product subject to conformity assessment  (for example, each manufactured item will have a different value from the other items)

Measurement variation

  • Variable yspecific: Apparent variation in product, due to overall limited measurement quality when determining the value of the quality characteristic of one specific item of the product subject to conformity assessment
  • Variable yglobal: Apparent variation in product, due to overall limited measurement quality when determining the values of the quality characteristic of a population of items of the product subject to conformity assessment (e.g. limited sampling)
  • Measurement instrument variation: Component in either yspecific or yglobal arising from limited quality of the measurement instrument used in assessing conformity of the product
  • Measurement standard variation: Component in either yspecific or yglobal arising from limited quality of the measurement standard used in assessing conformity of the product

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