Metrology is the science of measurement.

According to the European Union, it covers three main activities :

  • The definition of internationally accepted units of measurement, e.g. the meter.
  • The realization of units of measurement by scientific methods, e.g. the realization of a meter through the use of lasers.
  • The establishment of traceability chains by determining and documenting the value and accuracy of a measurement and disseminating that knowledge.

It is considered in three categories with different levels of complexity and accuracy :

  1. Scientific metrology deals with the organization and development of measurement standards and with their maintenance (highest level).
  2. Industrial metrology has to ensure the adequate functioning of measurement instruments used in industry as well as in production and testing processes.
  3. Legal metrology is concerned with measurements where these influence the transparency of economic transactions, health and safety.

Fundamental metrology has no international definition, but it signifies the highest level of accuracy within a given field. It may therefore be described as the top-level branch of scientific metrology.

In Six Sigma, there are four parameters in metrology:

  • Precision: It is the range of a parameter and can be measured as a range that can be called cp.
  • Bias: It is the deviation of the average of a parameter from the target value. It is the noncentering average minus the design center.
  • Accuracy: It combines precision and bias (cpx) if tied to a specification width.
  • Discrimination: It is the ratio of product spread to measurement spread, with a minimum ratio of 5:1.

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