2. Measurement Process Characterization
2.5. Uncertainty analysis

2.5.8.

Treatment of uncorrected bias

Background

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 (Turgel and Vecchia).

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):

1. The final uncertainty must be greater than or equal to the uncertainty that would be quoted if the bias were corrected.
2. The final uncertainty must reduce to the same uncertainty given that the bias correction is applied.
3. 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.
4. The method should be transferable so that both the uncertainty and the bias can be used as components of uncertainty in another uncertainty statement.
5. The method should be easy to implement.