A signed linear chain
For y = A − B, the nominal dimensions subtract, but symmetric worst-case half-widths add: if A = 10 ± 0.1 and B = 4 ± 0.2, y = 6 ± 0.3. The limits are 5.7–6.3 in the chosen unit.
RSS needs a statistical interpretation
Root-sum-square is meaningful as spread when contributions and their statistical scale are known and covariance is absent or handled. A drawing half-tolerance does not automatically equal three standard deviations.
Model completeness matters
Worst-case interval arithmetic bounds the specified linear model. It does not cover omitted dimensions, form error, thermal change or assembly deformation, and it is not an unconditional guarantee of physical assembly.
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Sources and review scope
Reviewed 23 October 2026. Arithmetic examples are labelled separately from source-table facts and standard scope.
NIST TN 1297 Appendix A
Equation A-3 includes sensitivity coefficients and covariance. Linear RSS here is a mathematical application, not a manufacturing acceptance rule.
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