The sum of the squares of the standard deviations, expressing the contributions to the overall precision from various sources of uncertainty.
Sources:
Orange Book, 2nd ed., p. 5 (https://media.iupac.org/publications/analytical_compendium/)
PAC, 1990, 62, 2167. 'Glossary of atmospheric chemistry terms (Recommendations 1990)' on page 2218 (https://doi.org/10.1351/pac199062112167)
PAC, 1994, 66, 595. 'Nomenclature for the presentation of results of chemical analysis (IUPAC Recommendations 1994)' on page 601 (https://doi.org/10.1351/pac199466030595)