Central to all rigorous and economical inspection is the requirement that variables measures be expressed through operational definitions. Such definitions demand that we express the measurement in terms of:
When we commit ourselves to a contract with a supplier or customer it is essential that we do not allow ambiguity over the meaning of the terms of the contract. In 1970 the West Gate Bridge near Melbourne, Australia collapsed. The enquiry into the failure found that the engineers had specified the supply of a quantity of "flat steel plate". Through using words such as "flat", rather than a specification of allowed distortion:
Measurement processes for use in categorising items must be stable.
When variables data is used for classifying parts, the effects of measurement-process variation tend to "fuzz" the specification limits. In order to assess the magnitude and importance of this effect, the standard error (known as standard uncertainty in ISO documents) and probable error of the measurement process must be known. Full details of how to handle this are described at p331 of Wheeler & Chambers (1992).
This page last updated 13th July 2001
Copyright ©2001 A N Cutler