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Dynamics Models (dynamics + models)

Distribution by Scientific Domains

Life Sciences	25%

Selected Abstracts

Score Tests for Exploring Complex Models: Application to HIV Dynamics Models

BIOMETRICAL JOURNAL, Issue 1 2010
Julia Drylewicz
Abstract In biostatistics, more and more complex models are being developed. This is particularly the case in system biology. Fitting complex models can be very time-consuming, since many models often have to be explored. Among the possibilities are the introduction of explanatory variables and the determination of random effects. The particularity of this use of the score test is that the null hypothesis is not itself very simple; typically, some random effects may be present under the null hypothesis. Moreover, the information matrix cannot be computed, but only an approximation based on the score. This article examines this situation with the specific example of HIV dynamics models. We examine the score test statistics for testing the effect of explanatory variables and the variance of random effect in this complex situation. We study type I errors and the statistical powers of this score test statistics and we apply the score test approach to a real data set of HIV-infected patients. [source]

Data cloning: easy maximum likelihood estimation for complex ecological models using Bayesian Markov chain Monte Carlo methods

ECOLOGY LETTERS, Issue 7 2007
Subhash R. Lele
Abstract We introduce a new statistical computing method, called data cloning, to calculate maximum likelihood estimates and their standard errors for complex ecological models. Although the method uses the Bayesian framework and exploits the computational simplicity of the Markov chain Monte Carlo (MCMC) algorithms, it provides valid frequentist inferences such as the maximum likelihood estimates and their standard errors. The inferences are completely invariant to the choice of the prior distributions and therefore avoid the inherent subjectivity of the Bayesian approach. The data cloning method is easily implemented using standard MCMC software. Data cloning is particularly useful for analysing ecological situations in which hierarchical statistical models, such as state-space models and mixed effects models, are appropriate. We illustrate the method by fitting two nonlinear population dynamics models to data in the presence of process and observation noise. [source]

Depensation: evidence, models and implications

FISH AND FISHERIES, Issue 1 2001
Liermann
We review the evidence supporting depensation, describe models of two depensatory mechanisms and how they can be included in population dynamics models and discuss the implications of depensation. The evidence for depensation can be grouped into four mechanisms: reduced probability of fertilisation, impaired group dynamics, conditioning of the environment and predator saturation. Examples of these mechanisms come from a broad range of species including fishes, arthropods, birds, mammals and plants. Despite the large number of studies supporting depensatory mechanisms, there is very little evidence of depensation that is strong enough to be important in a population's dynamics. However, because factors such as demographic and environmental variability make depensatory population dynamics difficult to detect, this lack of evidence should not be interpreted as evidence that depensatory dynamics are rare and unimportant. The majority of depensatory models are based on reduced probability of fertilisation and predator saturation. We review the models of these mechanisms and different ways they can be incorporated in population dynamics models. Finally, we discuss how depensation may affect optimal harvesting, pest control and population viability analysis. [source]

Does population ecology have general laws?

OIKOS, Issue 1 2001
Peter Turchin
There is a widespread opinion among ecologists that ecology lacks general laws. In this paper I argue that this opinion is mistaken. Taking the case of population dynamics, I point out that there are several very general law-like propositions that provide the theoretical basis for most population dynamics models that were developed to address specific issues. Some of these foundational principles, like the law of exponential growth, are logically very similar to certain laws of physics (Newton's law of inertia, for example, is almost a direct analogue of exponential growth). I discuss two other principles (population self-limitation and resource-consumer oscillations), as well as the more elementary postulates that underlie them. None of the "laws" that I propose for population ecology are new. Collectively ecologists have been using these general principles in guiding development of their models and experiments since the days of Lotka, Volterra, and Gause. [source]

Self-organized regular surface patterning by pulsed laser ablation

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 3 2009
Juergen Reif
Abstract The impact of intense ultra short laser pulses on solid surface - as in laser ablation - results in a transient perturbation of the material to a state far from equilibrium. Due to ultrafast relaxation of the transient disorder in a few picoseconds, self-organized surface patterns occur, with a typical feature size at the order of 100 nm or less, similar as observed in ion sputtering and explained by non-linear dynamics models. The feature size of these structures is determined by the deposited energy dose, their shape and orientation crucially depends on the state of polarization of the incident light. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

System Dynamics as a Structural Theory in Operations Management

PRODUCTION AND OPERATIONS MANAGEMENT, Issue 3 2008
Andreas Größler
The purpose of the paper is to demonstrate the usefulness of (1) system dynamics as a structural theory for operations management and (2) system dynamics models as content theories in operations management. The key findings are that, although feedback loops, accumulation processes, and delays exist and are widespread in operations management, often these phenomena are ignored completely or not considered appropriately. Hence, it is reasoned why system dynamics is well suited as an approach for many operations management studies, and it is shown how system dynamics theory can be used to explain, analyze, and understand such phenomena in operations management. The discussion is based on a literature review and on conceptual considerations, with examples of operations management studies based on system dynamics. Implications of using this theory include the necessary re-framing of some operations management issues and the extension of empirical studies by dynamic modeling and simulation. The value of the paper lies in the conceptualization of the link between system dynamics and operations management, which is discussed on the level of theory. [source]

SURVIVAL AND GROWTH IN RETAIL AND SERVICE INDUSTRIES: EVIDENCE FROM FRANCHISED CHAINS§

THE JOURNAL OF INDUSTRIAL ECONOMICS, Issue 3 2010
RENÁTA KOSOVÁ
Using data on franchised chains, which are the type of single-product entities emphasized in industry dynamics models, we show that age and size affect growth and survival even after controlling for chain characteristics and unobserved chain-specific efficiency. This implies that age and size affect firm growth and survival for reasons other than those emphasized in learning-type models. We also find that several chain characteristics affect growth and survival directly, and thus controlling for firm characteristics is important. Finally, we find that chain size increases rather than decreases exit among young chains, and chains converge in size over time. [source]