Heterogeneities in a DU yield sampling errors. In terms of Gy’s principles, these errors include FE and GSE, and they apply to all sampling methods (e.g., discrete, composite, and ISM). As a result, measurements of concentrations based on those collected sample(s) may not be representative (i.e., yield an unbiased estimate of the true mean concentration of the contaminant present in the DU). The magnitude of the heterogeneities can be expected to vary among DUs. The terms “homogeneous DU” and “heterogeneous DU” are used in this document to qualitatively describe the relative magnitudes of compositional and distributional heterogeneities (CH and DH).