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Particular Models (particular + models)

Distribution by Scientific Domains
Mathematics and Statistics33%

Selected Abstracts

The maximum entropy formalism and the idiosyncratic theory of biodiversity

ECOLOGY LETTERS, Issue 11 2007
Salvador Pueyo
Abstract Why does the neutral theory, which is based on unrealistic assumptions, predict diversity patterns so accurately? Answering questions like this requires a radical change in the way we tackle them. The large number of degrees of freedom of ecosystems pose a fundamental obstacle to mechanistic modelling. However, there are tools of statistical physics, such as the maximum entropy formalism (MaxEnt), that allow transcending particular models to simultaneously work with immense families of models with different rules and parameters, sharing only well-established features. We applied MaxEnt allowing species to be ecologically idiosyncratic, instead of constraining them to be equivalent as the neutral theory does. The answer we found is that neutral models are just a subset of the majority of plausible models that lead to the same patterns. Small variations in these patterns naturally lead to the main classical species abundance distributions, which are thus unified in a single framework. [source]

A general formulation of hypoplasticity

INTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 15 2004
J. Lanier
Abstract A general formulation of the theory of hypoplasticity is given, under the assumption that Cauchy stress is the only state variable. Such a formulation allows to discuss the relation between the so-called out-of-axis and in-axis moduli. It is proved that, in general, the out-of-axis moduli are independent from the in-axis moduli, which allows the former to be calibrated making use of experimental shear band data, as it is done in CLoE-hypoplasticity. The implicit assumptions made in K-hypoplasticity are detailed for two particular models of this family. Copyright © 2004 John Wiley & Sons, Ltd. [source]

A new approach to anisotropic, bounding surface plasticity: general formulation and simulations of natural and reconstituted clay behaviour

INTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 3 2001
A. Gajo
Abstract It is demonstrated that an efficient framework for the description of a two surface kinematic hardening/bounding surface plasticity model can be devised if the model is conceived in a normalized stress space in which the bounding surface remains of constant size. The hardening of the bounding surface is contained in the transformation laws linking ,real' and ,normalized' stresses while the geometric rules controlling kinematic hardening are considered only in terms of normalized stresses. Within the general framework thus constructed, two particular models are developed in outline. Owing to the proposed anisotropic hardening, these models are shown to have the ability to reproduce the observed characteristics of the progressive destructuration of natural cemented clays and of the plastic anisotropy of reconstituted soils. Copyright © 2001 John Wiley & Sons, Ltd. [source]

The Effect of the Estimation on Goodness-of-Fit Tests in Time Series Models

JOURNAL OF TIME SERIES ANALYSIS, Issue 4 2005
Yue Fang
Abstract., We analyze, by simulation, the finite-sample properties of goodness-of-fit tests based on residual autocorrelation coefficients (simple and partial) obtained using different estimators frequently used in the analysis of autoregressive moving-average time-series models. The estimators considered are unconditional least squares, maximum likelihood and conditional least squares. The results suggest that although the tests based on these estimators are asymptotically equivalent for particular models and parameter values, their sampling properties for samples of the size commonly found in economic applications can differ substantially, because of differences in both finite-sample estimation efficiencies and residual regeneration methods. [source]

Household Heterogeneity and Real Exchange Rates,

THE ECONOMIC JOURNAL, Issue 519 2007
Narayana R. Kocherlakota
We assume that individuals can fully insure themselves against cross-country shocks but not against individual-specific shocks. We consider two particular models of limited risk-sharing: domestically incomplete markets (DI) and private information,Pareto optimal (PIPO) risk-sharing. For each model, we derive a restriction relating the cross-sectional distributions of consumption and real exchange rates. We evaluate these restrictions using household-level consumption data from the US and the UK. We show that the PIPO restriction fits the data well when households have a coefficient of relative risk aversion of around 5. The restrictions implied by the complete risk-sharing model and the DI model fare poorly. [source]

Maximum Likelihood Estimation in Dynamical Models of HIV

BIOMETRICS, Issue 4 2007
J. Guedj
Summary The study of dynamical models of HIV infection, based on a system of nonlinear ordinary differential equations (ODE), has considerably improved the knowledge of its pathogenesis. While the first models used simplified ODE systems and analyzed each patient separately, recent works dealt with inference in non-simplified models borrowing strength from the whole sample. The complexity of these models leads to great difficulties for inference and only the Bayesian approach has been attempted by now. We propose a full likelihood inference, adapting a Newton-like algorithm for these particular models. We consider a relatively complex ODE model for HIV infection and a model for the observations including the issue of detection limits. We apply this approach to the analysis of a clinical trial of antiretroviral therapy (ALBI ANRS 070) and we show that the whole algorithm works well in a simulation study. [source]