The standard deviation sum-of-fractions for surface soil is calculated as follows:
\begin{equation} S_{\text{sof}} = \sqrt{\Bigg(\dfrac{\text{Max}[S_\text{su1},S_\text{ref1}]}{DCGL_1}\Bigg)^2 + \Bigg(\dfrac{\text{Max}[S_\text{su2}, S_\text{ref2}]}{DCGL_2}\Bigg)^2 + etc.}\end{equation}
Where the Derived Concentration Guidance Levels (DCGL) are known and the standard deviations (\(S\)) values are estimated for each nuclide in the survey unit and the reference area.