6,300 research outputs found

    Validation of Soft Classification Models using Partial Class Memberships: An Extended Concept of Sensitivity & Co. applied to the Grading of Astrocytoma Tissues

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    We use partial class memberships in soft classification to model uncertain labelling and mixtures of classes. Partial class memberships are not restricted to predictions, but may also occur in reference labels (ground truth, gold standard diagnosis) for training and validation data. Classifier performance is usually expressed as fractions of the confusion matrix, such as sensitivity, specificity, negative and positive predictive values. We extend this concept to soft classification and discuss the bias and variance properties of the extended performance measures. Ambiguity in reference labels translates to differences between best-case, expected and worst-case performance. We show a second set of measures comparing expected and ideal performance which is closely related to regression performance, namely the root mean squared error RMSE and the mean absolute error MAE. All calculations apply to classical crisp classification as well as to soft classification (partial class memberships and/or one-class classifiers). The proposed performance measures allow to test classifiers with actual borderline cases. In addition, hardening of e.g. posterior probabilities into class labels is not necessary, avoiding the corresponding information loss and increase in variance. We implement the proposed performance measures in the R package "softclassval", which is available from CRAN and at http://softclassval.r-forge.r-project.org. Our reasoning as well as the importance of partial memberships for chemometric classification is illustrated by a real-word application: astrocytoma brain tumor tissue grading (80 patients, 37000 spectra) for finding surgical excision borders. As borderline cases are the actual target of the analytical technique, samples which are diagnosed to be borderline cases must be included in the validation.Comment: The manuscript is accepted for publication in Chemometrics and Intelligent Laboratory Systems. Supplementary figures and tables are at the end of the pd

    A time-dependent anisotropic model for argillaceous rocks: application to an underground excavation in Callovo-Oxfordian claystone

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    The paper presents a constitutive model for argillaceous rocks, developed within the framework of elastoplasticity, that includes a number of features that are relevant for a satisfactory description of their hydromechanical behaviour: anisotropy of strength and stiffness, behaviour nonlinearity and occurrence of plastic strains prior to peak strength, significant softening after peak, time-dependent creep deformations and permeability increase due to damage. Both saturated and unsaturated conditions are envisaged. The constitutive model is then applied to the simulation of triaxial and creep tests on Callovo-Oxfordian (COx) claystone. Although the main objective has been the simulation of the COx claystone behaviour, the model can be readily used for other argillaceous materials. The constitutive model developed is then applied, via a suitable coupled hydromechanical formulation, to the analysis of the excavation of a drift in the Meuse/Haute-Marne Underground Research Laboratory. The pattern of observed pore water pressures and displacements, as well as the shape and extent of the damaged zone, are generally satisfactorily reproduced. The relevance and importance of rock anisotropy and of the development of a damaged zone around the excavations are clearly demonstrated.Peer ReviewedPostprint (author's final draft

    Unraveling the temperature dependence of the yield strength in single-crystal tungsten using atomistically-informed crystal plasticity calculations

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    We use a physically-based crystal plasticity model to predict the yield strength of body-centered cubic (bcc) tungsten single crystals subjected to uniaxial loading. Our model captures the thermally-activated character of screw dislocation motion and full non-Schmid effects, both of which are known to play a critical role in bcc plasticity. The model uses atomistic calculations as the sole source of constitutive information, with no parameter fitting of any kind to experimental data. Our results are in excellent agreement with experimental measurements of the yield stress as a function of temperature for a number of loading orientations. The validated methodology is then employed to calculate the temperature and strain-rate dependence of the yield strength for 231 crystallographic orientations within the standard stereographic triangle. We extract the strain-rate sensitivity of W crystals at different temperatures, and finish with the calculation of yield surfaces under biaxial loading conditions that can be used to define effective yield criteria for engineering design models

    Wall Adhesion and Constitutive Modelling of Strong Colloidal Gels

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    Wall adhesion effects during batch sedimentation of strongly flocculated colloidal gels are commonly assumed to be negligible. In this study in-situ measurements of colloidal gel rheology and solids volume fraction distribution suggest the contrary, where significant wall adhesion effects are observed in a 110mm diameter settling column. We develop and validate a mathematical model for the equilibrium stress state in the presence of wall adhesion under both viscoplastic and viscoelastic constitutive models. These formulations highlight fundamental issues regarding the constitutive modeling of colloidal gels, specifically the relative utility and validity of viscoplastic and viscoelastic rheological models under arbitrary tensorial loadings. The developed model is validated against experimental data, which points toward a novel method to estimate the shear and compressive yield strength of strongly flocculated colloidal gels from a series of equilibrium solids volume fraction profiles over various column widths.Comment: 37 pages, 12 figures, submitted to Journal of Rheolog

    Modelling Creep Behaviour of Superheater Materials

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    AbstractThe energy demand of human being is ever increasing. The naturally available energy resources are in a crude form and need conversion to one which is readily available for end use. Power plants play the role of this conversion process. Majority of the conversion processes take place at severe conditions of very high temperature and high pressure. Hence, power plant components always exhibit inelastic behaviours like creep and fatigue. The design of such components should take these inelastic behaviours in to consideration. This work focuses on modelling the creep behaviour of superheater materials. Specifically, creep constitutive model of T91 steel which is commonly used for constructing superheater tubes is developed and validated with results from experimental work. Then a material user subroutine has been written to incorporate the model in commercial software ABAQUS

    Can EU Conditionality Remedy Soft Budget Constraints in Transition Countries?

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    Soft budget constraints (SBCs) are a persistent feature of transition economies and have been blamed for i.a. a lack of fiscal consolidation and sluggish growth. EU eastward enlargement has - among other things - been conditioned on tackling SBCs. This paper analyzes such outside conditionality theoretically and empirically. First, modelling the SBC problem as a war of attrition between the applicant countries' governments and firms we find that outside conditionality can foster SBC hardening. Yet, toughening the EU stance or reducing the number of enlargement rounds may have ambiguous effects. Second, estimating SBC hardening in a partial adjustment model by measuring the reaction of employment to output changes we find that EU conditionality did indeed help candidates to fight SBCs.soft budget constraint, EU enlargement, war of attrition

    Does Fiscal Discipline towards Sub-national Governments Affect Citizens’ Well-being? Evidence on Health

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    This paper aims at assessing the impact of fiscal discipline towards sub-national governments on citizens’ well-being. We model fiscal discipline by considering the expectations of deficit bailouts by Central Government, and focus on a particular dimension of well-being, namely health outcomes at the regional level. We study then how bailout expectations affect the expenditure for health care policies carried out by Regional Governments: in the presence of opportunistic behaviours by local governments – induced by soft budget constraints – bailout expectations should affect only spending inefficiency, and should not have any real effects on citizens’ health. To investigate this issue, we model the efficient use of public resources for health care delivery as an input requirement frontier, and assess the effects of bailout expectations on both the structural component of health spending and its deviations from the best practice. The evidence from a sample of 15 Italian Regions observed from 1993 to 2006 highlights that bailout expectations do not significantly influence the position of the frontier, thus do not affect citizens’ health. However, they appear to exert a remarkable impact on excess spending.Intergovernmental relationships, Soft budget constraint, Bailout expectations, Health care policy, Spending efficiency

    Meta-level performance management of simulation: The problem context retrieval approach

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    This paper introduces an approach for meta-level performance management of simulation. The paper focuses on the efficiency of modelling and simulation of organisational ICT and related BP systems. The analysis of these systems is essential for the analysis and design of ERP (Enterprise Resource Planning) systems. Projects initiated for the simulation of these systems frequently lead to the use of complex models with a high computing capacity requirement (showing the importance of efficiency) and typically, they are executed in a changing problem context environment. The performance management approach proposed by the paper provides methodological support for problem context transitions based on the models of changing problem contexts and on the set of efficiency principles (taking into account the efficiency of transitions too) together with a formal information retrieval based model of efficiency. This efficiency model takes into account both short- and long-term efficiency requirements, namely the efficiency of each step and the efficiency of the whole simulation process
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