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Do limits make sense for qualitative characteristics?

12 November 2012: Edgar Dietrich

Depending on the standard, standard of technical associations or company guidelines, there are different statistics to evaluate test or measurement processes in measurement process capability analyses. These statistics are compared to specified limits in order to evaluate capability. The limits discussed in the following refer to quantitative characteristics. It is also assumed that the procedures and the calculation of statistics are known.

Do limits make sense?

The first question that comes to mind is whether limits make sense. The clear answer is “yes” in case most of the measurement processes can be evaluated based on these limits since these limits provide clear conditions. Many practitioners confirm that the widely used procedures of the measurement process capability analysis can be applied to more than 50% of applications. This is a considerable amount. However, it is also clear that most measurement and test processes relevant in practice cannot be evaluated one-to-one with this procedure and these limits. It depends on the characteristic to be tested but also on the complexity of the measurement process under real conditions. In many cases you cannot even distinguish between a manufacturing and measurement process. For this reasons, it is important to evaluate the respective conditions in such cases and to establish them according to the current state of the art. You individually decide on capability by means of technological and economic aspects and by considering the respective risk.

Important limits and their meaning

By ignoring the procedure and calculation according to GUM (Guide to the Expression of Uncertainty in Measurement) in terms of measurement process capability, you may divide the evaluation procedures in three categories:

  • Company guidelines for the evaluation of measurement processes
  • MSA (measurement systems analysis) of the AIAG (automotive industry action group)
  • VDA 5 Capability of Measurement Processes or ISO 22514-7 Measurement Process Capability

It is always the own guideline that is relevant to each company at first. If such a guideline does not exist, superior standards of technical associations or general standards will be used in audits. You may divide company guidelines in two categories...