Measurement Process Characterization
Treatment of uncorrected bias
The ISO Guide ( ISO) for
expressing measurement uncertainties assumes that all biases are
corrected and that the uncertainty applies to the corrected result.
For measurements at the factory floor level, this approach has
several disadvantages. It may not be practical, may be expensive
and may not be economically sound to correct for biases that do not
impact the commercial value of the product
Reasons for not correcting for bias
Corrections may be expensive to implement if they require
modifications to existing software and "paper and pencil"
corrections can be both time consuming and prone to error. In the
scientific or metrology laboratory, biases may be documented in
certain situations, but the mechanism that causes the bias may not
be fully understood, or repeatable, which makes it difficult to
argue for correction. In these cases, the best course of action
is to report the measurement as taken and adjust the uncertainty to
account for the "bias".
The question is how to adjust the uncertainty
A method needs to be developed which assures that the resulting
uncertainty has the following properties
(Phillips and Eberhardt):
- The final uncertainty must be greater than or equal to the
uncertainty that would be quoted if the bias were corrected.
- The final uncertainty must reduce to the same uncertainty
given that the bias correction is applied.
- The level of coverage that is achieved by the final
uncertainty statement should be at least the level obtained
for the case of corrected bias.
- The method should be transferable so that both the
uncertainty and the bias can be used as components of
uncertainty in another uncertainty statement.
- The method should be easy to implement.