The mean sum-of-fractions for surface soil is calculated as follows:
\begin{equation} Mean_{\text{sof}} = \dfrac{Mean_\text{su1}-Mean_\text{ref1}}{DCGL_1} + \dfrac{Mean_\text{su2}-Mean_\text{ref2}}{DCGL_2} + etc.\end{equation}
Where the Derived Concentration Guidance Levels (DCGL) are known and the mean values are estimated for each nuclide in the survey unit and the reference area.