When conclusions are drawn from measurement results, the uncertainty of the measurements must not be forgotten. This is particularly important when measurements are used to test whether or not a specification has been met.

Four cases of how a measurement result and its uncertainty may lie relative to the limits of a stated specification. (Similarly, an uncertainty may overlap the lower limit of a specification.)

In Case (a), both the result and the uncertainty fall inside the specified limits. This is classed as a ‘compliance’.

In Case (d), neither the result nor any part of the uncertainty band falls within the specified limits. This is classed as a ‘non-compliance’.

Cases (b) and (c) are neither completely inside nor outside the limits. No firm conclusion about compliance can be made.

Before stating compliance with a specification, always check the specification. Sometimes a specification covers various properties such as appearance, electrical connections, interchangeability, etc, which have nothing to do with what has been measured.

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Good practice online modules

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• Measurement uncertainty
• Introducing uncertainty
• Basics of uncertainty analysis