Effective degrees of freedom and approximate coverage
For independent input standard-uncertainty estimates in a linearized model, Welch–Satterthwaite gives ν_eff=u_c⁴/Σ_i[(c_i u_i)⁴/ν_i].
Key knowledge
For independent input standard-uncertainty estimates in a linearized model, Welch–Satterthwaite gives ν_eff=u_c⁴/Σ_i[(c_i u_i)⁴/ν_i]. When a near-normal output and this approximation are justified, a two-sided coverage factor is k=t_(1−α/2,ν_eff) and U=k u_c for approximate coverage 1−α.
Values and equations
ν_eff=u_c⁴/Σ_i[(c_i u_i)⁴/ν_i]; k=t_(1−α/2,ν_eff); U=ku_c
- u_c,U
- Combined/expanded output uncertainty; output quantity unit
- u_i
- Input standard uncertainty; input i unit
- c_i
- Output/input sensitivity; output unit per input unit
- ν_eff,ν_i
- Effective/component degrees of freedom
- k,α,t
- Coverage factor, excluded probability and Student-t quantile
Conditions and scope
- Independent variance components with stated ν_i, model sensitivities c_i and applicable normal/central-limit approximation; zero/infinite-degree terms use their limits.
- The expression is not a general formula for correlated or strongly nonnormal models. Type B is not automatically infinitely reliable; k=2 does not guarantee 95% coverage.
Sources · 1
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
Evidence and review
NIST TN 1297 (1994): Coverage factors: Appendix B.1–B.3, equation B-1 and explicit nonrigorous-approximation warning.
2026-10-10 · Independent AI editorial review: Independent root AI editorial review of all29 authored Measurement claims/conditions, displayed equations and37 origin relations. The original26 claims were read against41 primary source sections; all5 requested corrections and the3 affected source bindings were subsequently rechecked. Three actual scalar-nonlinear/vector/dynamic applications were read, independently differentiated or integrated, and checked against the specific additional source material described below. This is reference-content review, not human specialist certification, instrument certification or experimental replication. Full-field sign-off waits for the combined final conceptual matrix. Receipt: measurement-independent-content-review.1.
Entry details
Permanent link to this entry ↗- Record identity
- measurement-effective-degrees-coverage
- Revision
- university-core.1
- Structured reference record
- University core expansion