| 1. |
- Ólafsdóttir, Helga Kristín, 1990, et al.
(författare)
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Exact Gradients Improve Parameter Estimation in Nonlinear Mixed Effects Models with Stochastic Dynamics
- 2018
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Ingår i: Aaps Journal. - : Springer Science and Business Media LLC. - 1550-7416. ; 20:5
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Tidskriftsartikel (refereegranskat)abstract
- Nonlinear mixed effects (NLME) modeling based on stochastic differential equations (SDEs) have evolved into a promising approach for analysis of PK/PD data. SDE-NLME models go beyond the realm of standard population modeling as they consider stochastic dynamics, thereby introducing a probabilistic perspective on the state variables. This article presents a summary of the main contributions to SDE-NLME models found in the literature. The aims of this work were to develop an exact gradient version of the first-order conditional estimation (FOCE) method for SDE-NLME models and to investigate whether it enabled faster estimation and better gradient precision/accuracy compared to the use of gradients approximated by finite differences. A simulation-estimation study was set up whereby finite difference approximations of the gradients of each level were interchanged with the exact gradients. Following previous work, the uncertainty of the state variables was accounted for using the extended Kalman filter (EKF). The exact gradient FOCE method was implemented in Mathematica 11 and evaluated on SDE versions of three common PK/PD models. When finite difference gradients were replaced by exact gradients at both FOCE levels, relative runtimes improved between 6- and 32-fold, depending on model complexity. Additionally, gradient precision/accuracy was significantly better in the exact gradient case. We conclude that parameter estimation using FOCE with exact gradients can successfully be applied to SDE-NLME models.
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| 2. |
- Sverdrup, Harald, et al.
(författare)
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A System Dynamics Assessment of the Supply of Molybdenum and Rhenium Used for Super-alloys and Specialty Steels, Using the WORLD6 Model
- 2018
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Ingår i: BioPhysical Economics and Resource Quality. - : Springer Science and Business Media LLC. - 2366-0120 .- 2366-0112. ; 3:3
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Tidskriftsartikel (refereegranskat)abstract
- The extraction, supply, market price and recycling of the metals molybdenum and rhenium were modelled using an integrated system dynamics model. The resource estimates made here resulted in significantly larger estimates than earlier studies for molybdenum. Present molybdenum resources are about 75–80 million ton and about 7 million ton has been mined to date. The ultimately recoverable resources (URR) for molybdenum are about 65 million in primary resources and about 45 million ton in secondary sources, a total of about 111 million ton, and after considering technical extractability, evaluating several hundred different geological deposits, the extractable amount is about 90 million ton. For rhenium, URR is about 21,000 ton contained in mostly in molybdenum and copper, but some come from nickel, wolfram and platinum group metal ores. The model outputs show that molybdenum and rhenium are finite resources, and that they may become exhausted unless the degree of recycling will be significantly improved. Peak production is estimated to take place in 2060 for molybdenum and rhenium, with peak in stocks-in-use around 2090. The molybdenum and rhenium recycling rates are generally low. Both market intervention mechanisms and governance incentives should be used to increase recycling. The metal extraction and ore grades were modelled with good success when tested against observed data. The model predicts a significant decline in molybdenum supply after 2100 under the present demand combined with the present regime of recycling. The supply situation for rhenium is dependent on the situation applicable for molybdenum ore availability and rhenium recycling rate.
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| 3. |
- Ågerstrand, Marlene, et al.
(författare)
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A call for action : Improve reporting of research studies to increase the scientific basis for regulatory decision-making
- 2018
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Ingår i: Journal of Applied Toxicology. - : Wiley. - 0260-437X .- 1099-1263. ; 38:5, s. 783-785
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Tidskriftsartikel (refereegranskat)abstract
- This is a call for action to scientific journals to introduce reporting requirements for toxicity and ecotoxicity studies. Such reporting requirements will support the use of peer-reviewed research studies in regulatory decision-making. Moreover, this could improve the reliability and reproducibility of published studies in general and make better use of the resources spent in research.
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