Visualization technique for uncertainty budgets: Onion charts

A concise graphical depiction, the “onion” chart, is described for presenting uncertainty budgets for any measurand, y, having a set of component variances, . The onion chart consists of a concentric series of rings. Each ring is constructed from the set of fractional contributions, . The outer ring includes all and depicts the combined standard uncertainty, u _c (y), where u _c ²(y) ≡ ∑u _i ²(y). Moving inward, each successive ring deletes the largest remaining . A “reduced” is calculated for that ring from the remaining set of , with the fractional contributions calculated using the “reduced” . This format facilitates rapid visual comparison of the importance of each component, with minor components becoming visible as the larger components are successively deleted. An example is presented using data from pH metrology, including the physical interpretation of the resulting diagram.