Calculating Normalized Error Scores for Multi Facility Calibration Variance Analysis
Calculating normalized error scores isolates facility bias from measurement noise by evaluating raw offsets against combined expanded uncertainties.
En Score Calculation
Performance Comparison
Quantitative comparison used in proficiency testing evaluates the statistical difference between a participant’s measurement and a reference value relative to their expanded uncertainties. For ISO/IEC 17025 accredited laboratories, En score calculation provides an objective check of measurement competence. This evaluation determines if the discrepancy between the measured values falls within the boundaries of the combined uncertainty.
The score loses its validity if the participating laboratory fails to estimate its uncertainty in accordance with established metrological guidelines.
Statistical Formula
Calculating the score involves dividing the difference between the laboratory result and the reference value by the square root of the sum of their squared expanded uncertainties. An En score calculation result with an absolute value less than or equal to one indicates a satisfactory outcome, meaning the measurement is consistent with the reference value. A value greater than one requires corrective action.
This indicates that the laboratory has either a systematic error or an underestimated measurement uncertainty.
Uncertainty Interplay
Correctly estimating uncertainty is critical because an overestimation can hide poor measurement performance by artificially lowering the calculated score. Conversely, underestimating the uncertainty leads to an unsatisfactory score even when the measurement is accurate. The balance of these uncertainties is checked during the technical review of the proficiency test results.
This ensures that laboratories do not artificially inflate their declared uncertainties.
Compliance Verification
Laboratory auditors use these scores to verify that a calibration facility is capable of performing measurements within its scope of accreditation. If a facility receives a score indicating a failure, it must perform a root-cause analysis. The accreditation body will monitor the implementation of the corrective actions.