Coverage factors and degrees of freedom

Metra combines independent standard contributions using root sum of squares. Where finite component degrees of freedom are available, it calculates effective degrees of freedom using Welch-Satterthwaite.

The current policy uses fixed coverage factor k = 2. Expanded uncertainty is combined standard uncertainty multiplied by that factor. The result freezes the policy version, factor, effective degrees of freedom, and integrity hash. When all contributions have infinite degrees of freedom, the result records that explicitly.

Confidence-derived Student-t factors, variable coverage factors, covariance, and correlated components are outside the current policy.