| Abstract: | A recurring difficulty encountered in investigations of many metals and organic contaminants in ambient waters is that a substantial portion of water sample concentrations are below limits of detection established by analytical laboratories. Several methods were evaluated for estimating distributional parameters for such censored data sets using only uncensored observations. Their reliabilities were evaluated by a Monte Carlo experiment in which small samples were generated from a wide range of parent distributions and censored at varying levels. Eight methods were used to estimate the mean, standard deviation, median, and interquartile range. Criteria were developed, based on the distribution of uncensored observations, for determining the best performing parameter estimation method for any particular data set. The most robust method for minimizing error in censored-sample estimates of the four distributional parameters over all simulation conditions was the log-probability regression method. With this method, censored observations are assumed to follow the zero-to-censoring level portion of a lognormal distribution obtained by a least squares regression between logarithms of uncensored concentration observations and their z scores. |
| Genre: | Article |
| ProdID: | 70014951 |
| Citation Author: | Gilliom, R. J.; Helsel, D. R. |
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| Citation End Page: | 146 |
| Citation Issue: | 2 |
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| Citation Language: | English |
| Citation Larger Work Title: | Water Resources Research |
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| Citation Number Of Pages: | 12 |
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| Citation Search Results Text: | Estimation of distributional parameters for censored trace level water quality data. 1. Estimation techniques; 1986; Article; Journal; Water Resources Research; Gilliom, R. J.; Helsel, D. R. |
| Citation Start Page: | 135 |
| Citation Volume: | 22 |
| Citation Year: | 1986 |
| Type: | citation/reference |
| Text: | Estimation of distributional parameters for censored trace level water quality data. 1. Estimation techniques; 1986; Article; Journal; Water Resources Research; Gilliom, R. J.; Helsel, D. R. |
| URL (THUMBNAIL): | http://pubs.er.usgs.gov/thumbnails/outside_thumb.jpg |
| Date Other: | Wed, 1 Jan 1986 00:00 -0600 |
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