In a 2002 guidance manual for assessment of contaminated freshwater sediment, the US Environmental Protection Agency (EPA) listed [SEM] - [AVS] (a variant of the approach in which data are normalized by subtraction rather than division) as a "Moderate Priority" metric of ecosystem health.[21] Since 2005, EPA Equilibrium Partitioning Sediment Benchmarks (ESBs) for metal mixtures have been derived from a combination of Water Quality Criteria (WQC) and a variant of [SEM]/[AVS] corrected for the organic carbon content of the sediment.[22] In conducting its National Sediment Quality Survey, the EPA used [SEM] - [AVS] in its classification scheme; a sediment with an [SEM] - [AVS] concentration greater than 5 μg/g dry weight was classified as "Tier 1: Associated Adverse Effects on Aquatic Life or Human Health Are Probable."