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Data Reporting: Treatment of Outliers

Excerpted from the Southeastern Regional Implementation Manual (SERIM)

 7.4 Data Reporting and Statistics for Bioassay and Bioaccumulation Testing

 7.4.1 Definition and Treatment of Outliers

In most biological testing, some data points will be either much smaller or much larger then would be reasonably expected. Intuitively, outliers can be thought of as individual observations that are "far away" from the rest of the data. Outliers can be the result of faulty data, erroneous procedures, or invalid assumptions regarding the underlying distribution of all the data points that could potentially be sampled. In practice, a small number of outliers can be expected from a large number of samples including those that follow a normal distribution. Several techniques are available for outlier detection. Tests that involve hypothesis testing on data assumed to be normally distributed include Grubb's test, Rosner's test, and Dixon's test. The main advantage of using one of these formal statistical procedures is the ability to limit the risk of falsely flagging a valid data point as an "outlier".

When suspecting that a data point might be an outlier during the statistical analysis of bioassay and bioaccumulation data, the analysis should be performed twice, once with the suspected outlier and again without it. Both results should be reported and an explanation of why the outlier is believed to deserve exclusion or inclusion with the analysis should be presented. Such an explanation should not rely solely on the fact that some statistical test detected the outlier. In general, the more environmentally conservative approach should be utilized.

Citation: USEPA/USACE. 2008. Southeast Regional Implementation Manual (SERIM) for Requirements and Procedures for Evaluation of the Ocean Disposal of Dredged Material in South­eastern U.S. Atlantic and Gulf Coast Waters. EPA 904-B-08-001. U.S. Environmental Protection Agency Region 4 and U.S. Army Corps of Engineers, South Atlantic Division, Atlanta, GA. http://www.epa.gov/region4/water/oceans/documents/SERIM_Final_August 2008.pdf

 

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