Journal ArticleDOI
Estimation of Average Concentration in the Presence of Nondetectable Values
TLDR
In this article, the average concentration of a particular contaminant during some period of time, a certain proportion of the collected samples is often reported to be below the limit of detection.Abstract:
In the attempt to estimate the average concentration of a particular contaminant during some period of time, a certain proportion of the collected samples is often reported to be below the limit of detection. The statistical terminology for these results is known as censored data, i.e., nonzero values which cannot be measured but are known to be below some threshold. Samples taken over time are assumed to follow a lognormal distribution. Given this assumption, several techniques are presented for estimation of the average concentration from data containing nondetectable values. The techniques proposed include three methods of estimation with a left-censored lognormal distribution: a maximum likelihood statistical method and two methods involving the limit of detection. Each method is evaluated using computer simulation with respect to the bias associated with estimation of the mean and standard deviation. The maximum likelihood method was shown to produce unbiased estimates of both the mean and s...read more
Citations
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Journal ArticleDOI
Exposure of the U.S. population to bisphenol A and 4-tertiary-octylphenol: 2003-2004
TL;DR: Urine concentrations of total BPA differed by race/ethnicity, age, sex, and household income, and these first U.S. population representative concentration data for urinary BPA and tOP should help guide public health research priorities.
Journal ArticleDOI
Decrease in anogenital distance among male infants with prenatal phthalate exposure
Shanna H. Swan,Katharina M. Main,Fan Liu,Sara Stewart,Robin L. Kruse,Antonia M. Calafat,Catherine S. Mao,J. Bruce Redmon,Christine L. Ternand,S.J. Sullivan,J.Lynn Teague +10 more
TL;DR: The hypothesis that prenatal phthalate exposure at environmental levels can adversely affect male reproductive development in humans is supported.
Journal ArticleDOI
Polyfluoroalkyl chemicals in the U.S. population: data from the National Health and Nutrition Examination Survey (NHANES) 2003-2004 and comparisons with NHANES 1999-2000.
TL;DR: The apparent reductions in concentrations of PFOS, PFOA, and PFHxS most likely are related to discontinuation in 2002 of industrial production by electrochemical fluorination ofPFOS and related perfluorooctanesulfonyl fluoride compounds.
Journal ArticleDOI
Urinary levels of seven phthalate metabolites in the U.S. population from the National Health and Nutrition Examination Survey (NHANES) 1999-2000.
Manori J. Silva,Dana B. Barr,John A. Reidy,Nicole A. Malek,Carolyn C. Hodge,Samuel P. Caudill,John W. Brock,Larry L. Needham,Antonia M. Calafat +8 more
TL;DR: For example, this article measured the urinary monoester metabolites of seven commonly used phthalates in approximately 2,540 samples collected from participants of the National Health and Nutrition Examination Survey (NHANES), 1999-2000, who were greater than or equal to 6 years of age.
Journal ArticleDOI
Epidemiologic Evaluation of Measurement Data in the Presence of Detection Limits
Jay H. Lubin,Joanne S. Colt,David Camann,Scott Davis,James R. Cerhan,Richard K. Severson,Leslie Bernstein,Patricia Hartge +7 more
TL;DR: In this article, the authors consider the regression of an environmental measurement (dependent variable) on several covariates (independent variables) and find that the fill-in approach generally produces unbiased parameter estimates but may produce biased variance estimates and thereby distort inference when 30% or more of the data are below detection limits.
References
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Journal ArticleDOI
Tables for Maximum Likelihood Estimates: Singly Truncated and Singly Censored Samples
TL;DR: In this article, the maximum likelihood estimates of the mean and variance for simply truncated or simply censored samples drawn from a Normal distribution were derived, and a further worked example was provided.
Journal ArticleDOI
A New Mathematical Model of Air Pollutant Concentration Averaging Time and Frequency
TL;DR: A new mathematical model, described herein, gives mathematically sound results for all averaging times, and the two dominant features are derived from observations that indicate that air pollutant concentrations are approximately lognormally distributed for all pollutants in all cities for all averaged times.
Journal ArticleDOI
Procedures for Handling Aerometric Data
Gerald J. Nehls,Gerald G. Akland +1 more
TL;DR: In this paper, procedures for site characterization, method identification, data validation, and data summarization were established by the Environmental Protection Agency for the National Aerometric Data Bank (NADB).