Record Display for the EPA National Library Catalog


Main Title Maximum Likelihood Estimation of Water Quality Concentrations from Censored Data.
Author El-Shaarawi, A. H. ; Dolan, D. M. ;
CORP Author National Water Research Inst., Burlington (Ontario). ;International Joint Commission-United States and Canada, Windsor (Ontario). Great Lakes Regional Office.;Environmental Research Lab.-Duluth, MN.
Publisher c1989
Year Published 1989
Report Number EPA/600/J-89/434;
Stock Number PB90-264789
Additional Subjects Water quality ; Concentration(Composition) ; Lake Ontario ; Niagara River ; Detroit River ; Reprints ; Maximum likelihood estimation ; Lognormal distribution ; Censored data ; Trace amounts
Library Call Number Additional Info Location Last
NTIS  PB90-264789 Some EPA libraries have a fiche copy filed under the call number shown. 07/26/2022
Collation 9p
Concentration data for water quality parameters reported as less than the detection limit (DL) may be considered to have come from a censored probability distribution. Traditionally, if it was necessary to assign a value to a sample reported as less than DL, various ad hoc methods were used such as selecting arbitrary values (0, 1/2 DL, DL, etc.). As analytical methods improved, DL's became smaller and the choice of ad hoc methods had less of an effect. However, there are two cases where the choice of estimation method is still a problem: (1) it is desired to calculate a differential loading (i.e., downstream minus upstream); (2) a loading estimate is desired in a tributary whose flow rate is extremely large. Both cases apply in connecting channels such as the Niagara and Detroit Rivers. The background for development of a nonarbitrary method for handling these situations is presented and the resultant estimator is used on water quality data from the connecting channels and Lake Ontario. The equations for confidence intervals for estimated mean concentrations or loadings are also presented. The effect of changing the DL is discussed.