In this fourth article in his series on measurement uncertainty, Stephen MacDonald moves on to look at some of the lesser discussed points that need to be considered when calculating imprecision and its contribution to combined uncertainty.
The top-down method has simplified measurement uncertainty (MU) estimation. The laborious and statistically demanding bottom-up method is no longer the only approach. Studies have applied external quality assessment (EQA) data only to MU assessment.1 EQA provides different, very valuable insights, but generally it is not recommended for derivation of imprecision performance due to the smaller number of samples that are routinely available.
Imprecision quantified by internal quality control (IQC) minimises the risk of fewer uncertainty contributors being detected. IQC captures long-term imprecision, and the uncertainty contributors that cause it. However, the coefficient of variation (CV) of the IQC alone is not MU.2 Imprecision may contribute up to 50% of our overall MU budget, as seen in the previous article in this series (April 2024),3 and the convenience and accessibility of IQC data presents unseen challenges for the top-down method.4 For IQC to be a robust mechanism for determining MU, there are some assumptions (Table 1).
Assumptions for quantifying long term imprecision from IQC
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