RSS stands for root sum of squares: square each independent standard uncertainty contribution (already multiplied by its sensitivity coefficient), add them, then take the square root. Contributions of 0.6, 0.8 and 0.3 combine to about 1.04, far less than their straight sum of 1.7, because independent errors rarely all push the same way at once. RSS is valid only for uncorrelated terms; fully correlated terms add linearly instead.
Don't add them linearly, RSS the contributions and you're at 1.04 ppm, not 1.7.
'RSS it' and 'add it in quadrature' mean the same thing, and both are wrong when the terms share a common reference.
Also heard as
- root sum square
- quadrature sum
- adding in quadrature
Related terms
Combined standard uncertainty
The total standard uncertainty of a result, found by combining all input uncertainties through the measurement model.
Law of propagation of uncertainty
The first-order formula that combines input standard uncertainties, weighted by sensitivity coefficients, into the output uncertainty.
Correlation
A statistical link between two input quantities, so their errors move together and cannot be combined as independent.
Uncertainty budget
A table listing every uncertainty source in a measurement with its size, distribution, divisor and contribution to the total.