20 research outputs found

    Empirical and conditional likelihoods for two‐phase studies

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    Two‐phase, response‐dependent sampling is often used in regression settings that involve expensive covariate measurements. Conditional maximum likelihood (CML) is an attractive approach in many cases as it avoids modelling of the covariate distribution, unlike full maximum likelihood. Scott & Wild (2011) introduced an augmented CML approach which is semi‐parametric efficient in certain settings with a discrete response variable. We consider general regression models and show the Scott–Wild estimator of covariate effects has the same asymptotic efficiency as two empirical likelihood estimators, and that these estimators dominate the CML estimator. We compare the efficiencies of various estimators in simulation studies and illustrate the methodology in a two‐phase genetics study.RĂ©sumĂ©Un Ă©chantillonnage en deux phases dĂ©pendant de la rĂ©ponse est couramment utilisĂ© dans les contextes de rĂ©gression dont les covariables nĂ©cessitent des mesures coĂ»teuses. Le maximum de vraisemblance conditionnelle (MVC) constitue une approche intĂ©ressante pour de nombreux cas puisqu’il Ă©vite de modĂ©liser la distribution des covariables, contrairement au maximum de vraisemblance complĂšte. Scott et Wild (2011) ont proposĂ© une approche de MVC augmentĂ©e qui est semi‐paramĂ©triquement efficace dans certains contextes avec une variable rĂ©ponse discrĂšte. Les auteurs considĂšrent les modĂšles de rĂ©gression en gĂ©nĂ©ral et montrent que l’estimateur de Scott‐Wild (SW) pour l’effet des covariables offre la mĂȘme efficacitĂ© asymptotique que deux estimateurs issus de la vraisemblance empirique, et que ces estimateurs dominent celui du MVC. Les auteurs comparent l’efficacitĂ© de diffĂ©rents estimateurs dans des Ă©tudes de simulation et illustrent la mĂ©thodologie dans le cadre d’une Ă©tude gĂ©nĂ©tique Ă  deux phases.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/167757/1/cjs11566.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/167757/2/cjs11566_am.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/167757/3/cjs11566-sup-0001-Supinfo.pd

    Empirical and conditional likelihoods for two‐phase studies

    No full text
    Two‐phase, response‐dependent sampling is often used in regression settings that involve expensive covariate measurements. Conditional maximum likelihood (CML) is an attractive approach in many cases as it avoids modelling of the covariate distribution, unlike full maximum likelihood. Scott & Wild (2011) introduced an augmented CML approach which is semi‐parametric efficient in certain settings with a discrete response variable. We consider general regression models and show the Scott–Wild estimator of covariate effects has the same asymptotic efficiency as two empirical likelihood estimators, and that these estimators dominate the CML estimator. We compare the efficiencies of various estimators in simulation studies and illustrate the methodology in a two‐phase genetics study.RĂ©sumĂ©Un Ă©chantillonnage en deux phases dĂ©pendant de la rĂ©ponse est couramment utilisĂ© dans les contextes de rĂ©gression dont les covariables nĂ©cessitent des mesures coĂ»teuses. Le maximum de vraisemblance conditionnelle (MVC) constitue une approche intĂ©ressante pour de nombreux cas puisqu’il Ă©vite de modĂ©liser la distribution des covariables, contrairement au maximum de vraisemblance complĂšte. Scott et Wild (2011) ont proposĂ© une approche de MVC augmentĂ©e qui est semi‐paramĂ©triquement efficace dans certains contextes avec une variable rĂ©ponse discrĂšte. Les auteurs considĂšrent les modĂšles de rĂ©gression en gĂ©nĂ©ral et montrent que l’estimateur de Scott‐Wild (SW) pour l’effet des covariables offre la mĂȘme efficacitĂ© asymptotique que deux estimateurs issus de la vraisemblance empirique, et que ces estimateurs dominent celui du MVC. Les auteurs comparent l’efficacitĂ© de diffĂ©rents estimateurs dans des Ă©tudes de simulation et illustrent la mĂ©thodologie dans le cadre d’une Ă©tude gĂ©nĂ©tique Ă  deux phases.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/167757/1/cjs11566.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/167757/2/cjs11566_am.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/167757/3/cjs11566-sup-0001-Supinfo.pd

    Spatial Contamination and Potential Ecological Risk Assessment of Heavy Metals in Farmland Soil around Nonferrous Metal Smeltery in North China

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    Nonferrous metallurgy is an important source of heavy metal in the environment and consequently poses potential risks to ecosystems. The impact of smelting on the surrounding envi-ronment is a concern. In this work, the content levels of selected heavy metals—chromium (Cr), nickel (Ni), copper (Cu), zinc (Zn), lead (Pb), cadmium (Cd), and arsenic (As)—were investigated separately in soil samples collected around two nonferrous metal smelteries using inductively coupled plasma mass spectrometry (ICP-MS). The spatial distribution characteristics of soil metal pollutants was studied by ArcGIS methods and the potential ecological risks were assessed by the Hakanson potential eco-logical hazard index. The results show that soils were heavily polluted by Cr, Ni, Cu, Zn, Cd, Pb, and As. Their mean contents in soil around Smeltery A were 88, 62, 103, 1200, 1.4, 146, and 69 mg/kg, respectively, and those around Smeltery B were 86, 59, 83, 117, 0.53, 57, and 65 mg/kg, respectively. Their contents were obviously higher than the background values of soil Cr (68 mg/kg), Ni (31 mg/kg), Cu (22 mg/kg), Zn (78 mg/kg), Cd (0.09 mg/kg), Pb (22 mg/kg), and As (14 mg/kg). The distribution pattern in soil and risk assessment results show that the pollution surrounding the two smelteries reached intense and moderate ecological hazard and that the contribution of Cd and As was up to 87.05% and 82.59%, respectively. These results suggest that metal smelting makes a considerable contribution to soil pollution

    Spatial Contamination and Potential Ecological Risk Assessment of Heavy Metals in Farmland Soil around Nonferrous Metal Smeltery in North China

    No full text
    Nonferrous metallurgy is an important source of heavy metal in the environment and consequently poses potential risks to ecosystems. The impact of smelting on the surrounding envi-ronment is a concern. In this work, the content levels of selected heavy metals—chromium (Cr), nickel (Ni), copper (Cu), zinc (Zn), lead (Pb), cadmium (Cd), and arsenic (As)—were investigated separately in soil samples collected around two nonferrous metal smelteries using inductively coupled plasma mass spectrometry (ICP-MS). The spatial distribution characteristics of soil metal pollutants was studied by ArcGIS methods and the potential ecological risks were assessed by the Hakanson potential eco-logical hazard index. The results show that soils were heavily polluted by Cr, Ni, Cu, Zn, Cd, Pb, and As. Their mean contents in soil around Smeltery A were 88, 62, 103, 1200, 1.4, 146, and 69 mg/kg, respectively, and those around Smeltery B were 86, 59, 83, 117, 0.53, 57, and 65 mg/kg, respectively. Their contents were obviously higher than the background values of soil Cr (68 mg/kg), Ni (31 mg/kg), Cu (22 mg/kg), Zn (78 mg/kg), Cd (0.09 mg/kg), Pb (22 mg/kg), and As (14 mg/kg). The distribution pattern in soil and risk assessment results show that the pollution surrounding the two smelteries reached intense and moderate ecological hazard and that the contribution of Cd and As was up to 87.05% and 82.59%, respectively. These results suggest that metal smelting makes a considerable contribution to soil pollution
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