Search results for "Models"

Inferring the Relative Resilience of Alternative States

2013

Ecological systems may occur in alternative states that differ in ecological structures, functions and processes. Resilience is the measure of disturbance an ecological system can absorb before changing states. However, how the intrinsic structures and processes of systems that characterize their states affects their resilience remains unclear. We analyzed time series of phytoplankton communities at three sites in a floodplain in central Spain to assess the dominant frequencies or "temporal scales" in community dynamics and compared the patterns between a wet and a dry alternative state. The identified frequencies and cross-scale structures are expected to arise from positive feedbacks that…

Vertebrate Hedgehog is secreted on two types of extracellular vesicles with different signaling properties

2014

Hedgehog (Hh) is a secreted morphogen that elicits differentiation and patterning in developing tissues. Multiple proposed mechanisms to regulate Hh dispersion includes lipoprotein particles and exosomes. Here we report that vertebrate Sonic Hedgehog (Shh) is secreted on two types of extracellular-vesicles/exosomes, from human cell lines and primary chick notochord cells. Although largely overlapping in size as estimated from electron micrographs, the two exosomal fractions exhibited distinct protein and RNA composition. We have probed the functional properties of these vesicles using cell-based assays of Hh-elicited gene expression. Our results suggest that while both Shh-containing exo-ve…

ESE. Estudios sobre educación

2021

espanolEste estudio pretende avanzar en la comprension de la relacion entre el profesor y el estudiante universitario con el objetivo de mejorar el rendimiento de los estudiantes. Se articula sobre tres ejes: el enfoque de rendimiento del estudiante, la relacion entre diversos aspectos del profesor y, fi nalmente, el efecto de estos aspectos sobre el rendimiento del estudiante. Los resultados obtenidos de una muestra de 45 profesores y 932 estudiantes muestran que la orientacion al estudiante por parte del profesor afecta al aprendizaje percibido y a la satisfaccion del estudiante. Se alcanzan otros interesantes resultados que permiten establecer ciertas conclusiones e implicaciones. Englis…

The competency-based approach in developing countries : effects of curricular reforms in Sub-Saharan Africa

2013

For decades, all analyzes converge on the idea of the relative inefficiency of education systems in sub-Saharan Africa. Dropout, repetition, overstaffing, low quality of learning student failure and obsolescence of facilities, teaching materials and teachers, including sometimes the levels and quality of education remain somewhat uncertain, inadequate educational programs. In short, these are some of the ills that plague these systems and confirm the findings made by many reports. Given this weakness finding, educational authorities, supported by international aid mobilized in many countries to improve their systems to qualitatively, but also quantitatively equity because we must not forget…

Les langues, facteurs du rendement en lecture dans les évaluations PIRLS

2011

The starting point of this research is the question, may reading acquisition be more or less effective depending on the language in which it is perform? Two categories for classifying the languages have been developed. First the notion of linguistic family is employed to describe the languages from a cultural and historical perspective. Secondly, the notion of orthographic depth is used for differentiating the languages according to the correspondence between orthography and phonetic. These categories have been related to the databases PIRLS 2001 and 2006 (international assessments about reading developed by the IEA), the aim being to connect reading achievement to the language in which stu…

PISA 2000 results of reading literacy in Finland

2005

Predictability and prediction of lowest observed adverse effect levels in a structurally heterogeneous set of chemicals

2005

A database of chronic lowest observed adverse effect levels (LOAELs) for 234 compounds, previously compiled from different sources (Toxicology Letters79, 131-143 (1995)), was modelled using graph theoretical descriptors. This study reveals that data are not homogeneous. Only those data originating from the U.S. Environmental Protection Agency (EPA) reports could be well modelled by multilinear regression (MLR) and linear discriminant analysis (LDA). In contrast, data available from the specific procedures of the National Toxicology Program (NTP) database introduced noise and did not render good models either alone, or in combination with the EPA data.

Demyelination patterns in a mathematical model of multiple sclerosis.

2016

In this paper we derive a reaction-diffusion-chemotaxis model for the dynamics of multiple sclerosis. We focus on the early inflammatory phase of the disease characterized by activated local microglia, with the recruitment of a systemically activated immune response, and by oligodendrocyte apoptosis. The model consists of three equations describing the evolution of macrophages, cytokine and apoptotic oligodendrocytes. The main driving mechanism is the chemotactic motion of macrophages in response to a chemical gradient provided by the cytokines. Our model generalizes the system proposed by Calvez and Khonsari (Math Comput Model 47(7–8):726–742, 2008) and Khonsari and Calvez (PLos ONE 2(1):e…

The Multiscale Stochastic Model of Fractional Hereditary Materials (FHM)

2013

Abstract In a recent paper the authors proposed a mechanical model corresponding, exactly, to fractional hereditary materials (FHM). Fractional derivation index 13 E [0,1/2] corresponds to a mechanical model composed by a column of massless newtonian fluid resting on a bed of independent linear springs. Fractional derivation index 13 E [1/2, 1], corresponds, instead, to a mechanical model constituted by massless, shear-type elastic column resting on a bed of linear independent dashpots. The real-order of derivation is related to the exponent of the power-law decay of mechanical characteristics. In this paper the authors aim to introduce a multiscale fractance description of FHM in presence …

Poisson convergence on continuous time branching random walks and multistage carcinogenesis.

1982

A theorem for Poisson convergence on realizations of two-dimensional Branching Random Walks with an underlying continuous time Markov Branching Process is proved. This result can be used to gain an approximation for the number of cells having sustained a certain deficiency after a long time in multistage carcinogenesis.