Measurement Uncertainty (VDA 5 / GUM)

Measurement uncertainty condenses every dispersion contribution of a measurement process into one figure. Unlike the AIAG metrics, which rate individual effects as percentages, VDA 5 follows the GUM and builds an uncertainty budget: every contribution is captured as a standard uncertainty $u_i$ and added in quadrature.

The coverage factor $k=2$ corresponds to roughly 95 % coverage. The expanded uncertainty $U$ is then related to the tolerance:

• $Q_{MS}\le 15\%$ — capable
• $15\%<Q_{MS}\le 30\%$ — conditionally capable, check process reserve
• $Q_{MS}>30\%$ — not capable

Typical contributions are repeatability, comparison precision, resolution, the calibration uncertainty of the reference part, plus bias and linearity. The latter enter as $u_{BI}=\sqrt{{\max }_i|b_i{|}^2+(SE(a)\cdot \text{Range}{)}^2}$.