Table 2.

Four example-approaches to uncertainty estimation.

Approach	Force F, N Diameter D, mm	Random Dispersion of σ Results	Coverage Factor, k
I approach	Type B evaluation, based on the standard requirements regarding accuracy: F-force measurement accuracy 1%. Rectangular PDF 1 D-accuracy of the sample preparation 1 mm. Rectangular PDF	No evaluation. Approach based on the assumption that uncertainty relates to the accuracy of the direct measurements only (F and D)	k = 2 (sometimes inappropriate but often used).
II approach		Type A evaluation based on the current test results. 4 results, ν = 3 degrees of freedom.	k = t0.95(νeff), where νeff—value of tp from t-distribution with an effective degree of freedom νeff obtained from the Welch-Satterthwaite formula. p-fraction of the distribution
III approach		Type B evaluation, based on historical results (e.g., inter-laboratory tests–reproducibility). Normal PDF.	k = 2, based on the assumption that the sR component with normal distribution is dominant.
IV approach		Type A evaluation based on the current test results. Normal PDF	No k-factor, 95% coverage intervals obtained by the Monte Carlo method [35]

¹ Probability density function.