A Comparison of Different Statistical Methods for Addressing Censored Left Data in Temporal Trends Analysis of Pyrethroi
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A Comparison of Different Statistical Methods for Addressing Censored Left Data in Temporal Trends Analysis of Pyrethroids in a California Stream Lenwood W. Hall Jr.1 · Elgin Perry2 · Ronald D. Anderson1 Received: 11 August 2020 / Accepted: 3 October 2020 © Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract This study compared four different statistical methods, involving six estimation procedures, for addressing censored left data in measuring temporal trends of eight different pyrethroids measured in sediment from a 10-year data set in a residential California stream (Pleasant Grove Creek). The statistical methods used were: the Kaplan–Meier (km) method; the robust regression on order statistics (ros using normal and log normal distributions rosln); the maximum likelihood estimation (mlen using normal and log normal distributions mleln); and a substitution method (sub) using ½ the detection limit. For five of the eight pyrethroids (bifenthrin, cyfluthrin, cypermethrin, lambda-cyhalothrin, and permethrin), the six statistical methods generally agree, with one exception, that the data set exhibit significant declining trends. In the case of bifenthrin, the slight disagreement among statistical methods only occurred for the mleln estimate that did not show a significant declining trend, whereas the other five methods did. For deltamethrin, esfenvalerate, and fenpropathrin, all six statistical methods were in agreement showing no significant trends. Possible reasons for declining sediment concentrations of pyrethroids in Pleasant Grove Creek are urban label changes effective in 2012–2015 that reduced residential use, variable annual rainfall, and more responsible homeowner use based on outreach/education programs. Measurements of chemical trends in the aquatic environment often are constrained by the practical reality of laboratory analysis because it is technically challenging to confirm the complete absence of a chemical of interest (Gibbons and Coleman 2001). A chemical may be present at some unknown concentration that is below the end of the concentration range that a chemist is able to detect. Although the real concentration is unknown chemists can report the nonzero values representing the lowest concentration that can be detected accurately for any given analytical method that is used. These non-zero concentrations are often used for various statistical applications, even though the actual value can only be narrowed down to a range of possible concentrations ranging from zero to the reporting limit (Gilbert 1987).
* Lenwood W. Hall Jr. [email protected] 1
Wye Research and Education Center, Agricultural Experiment Station, College of Agriculture and Natural Resources, University of Maryland, Queenstown, MD, USA
Colonial Beach, USA
2
In the field of environmental chemistry, a detection limit is defined as the concentration of a chemical that is statistically greater than the concentration of a methods blank with a high level of confidence or the lowest level of a specific chemic
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