In environmental toxicology, effects range low (ERL) and effects range median (ERM) are measures of toxicity in marine sediment. They are used by public agencies in the United States in formulating guidelines in assessing toxicity hazards, in particular from trace metals or organic contaminants. The ERL and ERM measures are expressed as specific chemical concentrations of a toxic substance in sediment. The ERL indicates the concentration below which toxic effects are scarcely observed or predicted: the ERM indicates that above which effects are generally or always observed. They are derived from biological toxicity assays and synoptic sampling. The numerical values are incorporated in sediment quality guidelines (SQGs) that were developed by Long and Morgan for the National Oceanic and Atmospheric Administration (NOAA) National Status & Trends program as informal tools to evaluate whether a concentration of a contaminant in sediment might have toxicological effects. These guidelines are used for screening sediments for trace metals and organic contaminants. They are not regulatory criteria in any way and are not intended to be used as such. Derivation NOAA originally calculated ERL/ERMs using existing toxicity data compiled from completed toxicity assays with varying endpoints, including effects on commonly tested organisms, particularly at sensitive life stages. The process is considered a "weight of evidence approach", in which results are based on a large database of previously conducted studies. The studies used included synoptically collected sediment chemical analyses and toxicity effects data. Using data already collected ("data mining") has the advantage of being able to quickly and inexpensively make an assessment with a large dataset that would otherwise require much more time-consuming and costly specific toxicity assays. Compiled data sets include a variety of endpoints including mortality, reproduction, growth rate, and juvenile survival in sediment toxicity data sets for all organisms for which tests have been conducted. Studies are screened, and only those assays using standardized methods and resulting in significant effects are used for the determination of ERL/ERM guidelines. In summary, the key links between the compiled studies are the testing of a specific analyte - toxicity assays used are for sediment, and a significant effect must be determined. The data is arranged by ordering the concentrations from lowest to highest. After ranking, both the 10th and 50th percentile concentrations are determined over the range of endpoint concentrations. The 10th percentile of the ranked data is identified as the ERL, and is considered indicative