Measurement Uncertainty Calculation Program (KOLAS-G-001)
A measurement-uncertainty calculation program that automatically computes the combined standard uncertainty, Welch-Satterthwaite effective degrees of freedom, coverage factor k, and expanded uncertainty per GUM and KOLAS-G-001.
Measurement uncertainty: why an Excel sheet is risky
Measurement uncertainty quantifies the confidence interval of a test or calibration result, and KOLAS-accredited bodies must state the expanded uncertainty on the report. Many labs compute uncertainty in a different Excel sheet per parameter, where formulas get damaged, degree-of-freedom handling is missed, and no revision history is kept — making reproducibility hard to demonstrate at assessment.
NeoLab QMS's Measurement Uncertainty Center makes the calculation reproducible with input forms that standardize the GUM procedure, and links the result to reports.
GUM · KOLAS-G-001 calculation procedure
| Step | Content |
|---|---|
| 1. Defining the model equation | Functional relationship between measurand y and input quantities x y = f(x1, x2, ...) |
| 2. Standard uncertainty evaluation | Type A (repeated-measurement statistics) · Type B (calibration certificate, spec, resolution — distribution conversion) |
| 3. Sensitivity coefficients | Partial derivative c_i = ∂f/∂x_i is used to calculate each factor's contribution |
| 4. Combined standard uncertainty | u_c = √Σ(c_i·u_i)² (assumed uncorrelated) |
| 5. Effective degrees of freedom | ν_eff calculated using the Welch-Satterthwaite formula |
| 6. Expanded uncertainty | U = k·u_c, typically k=2 at approximately a 95% confidence level |
Key features
1. Type A and Type B evaluation
Enter repeated-measurement data and it auto-computes the standard deviation and degrees of freedom (Type A); for Type B factors such as calibration certificate, spec, and resolution, choose a rectangular or triangular distribution to convert to standard uncertainty.
2. Combination and effective degrees of freedom
The combined standard uncertainty is derived from each factor's sensitivity coefficient and standard uncertainty; when the degrees of freedom are small, the effective degrees of freedom are computed with the Welch-Satterthwaite formula to determine the coverage factor k from the t-distribution.
3. Expanded uncertainty and decisions
Calculates expanded uncertainty U = k·u_c, and supports pass/fail decisions per conformity decision rules (such as KOLAS-G-002) by comparing against the upper/lower specification limits.
4. Report linkage and history
The computed expanded uncertainty, confidence level, and k value are stated on the report through linkage, and the calculation history is preserved to satisfy reproducibility and information-system validation (7.11).
Together with reports and document control
Uncertainty calculation results carry through to the report in the Accredited test report issuing system, while calculation procedures and revision history are controlled by ISO 17025 document management software. If you would like to understand accreditation criteria overall, see the ISO/IEC 17025 primer article.
Frequently asked questions
How is measurement uncertainty calculated?
It follows GUM and KOLAS-G-001. It evaluates each factor's standard uncertainty as Type A or Type B, multiplies by sensitivity coefficients to obtain the combined standard uncertainty, then multiplies by coverage factor k to obtain the expanded uncertainty.
How is the coverage factor k determined?
At about 95% confidence, k=2 is usually used, but when the degrees of freedom are small, the effective degrees of freedom are computed via the Welch-Satterthwaite formula and k is determined from the t-distribution. NeoLab QMS handles this automatically.
What is the difference between Type A and Type B uncertainty?
Type A is evaluated from the statistics of repeated measurements (standard deviation, degrees of freedom); Type B from existing information such as calibration certificates, specifications, and resolution. Type B is converted to standard uncertainty according to its distribution (rectangular, triangular, etc.).
Do calculation results flow into reports automatically?
Yes. The computed expanded uncertainty, confidence level, and k value can be linked into the report. See it in a 14-day free trial.