Local and global consistency properties for student placement

In the context of resource allocation on the basis of priorities, Ergin (2002) identifies a necessary and sufficient condition on the priority structure such that the student-optimal stable mechanism satisfies a consistency principle. Ergin (2002) formulates consistency as a local property based on a fixed population of agents and fixed resources -- we refer to this condition as local consistency and to his condition on the priority structure as local acyclicity. We identify a related but stronger necessary and sufficient condition (unit acyclicity) on the priority structure such that the student-optimal stable mechanism satisfies a more standard global consistency property. Next, we provide necessary and sufficient conditions for the student-optimal stable mechanism to satisfy converse consistency principles. We identify a necessary and sufficient condition (local shift-freeness) on the priority structure such that the student-optimal stable mechanism satisfies local converse consistency. Interestingly, local acyclicity implies local shift-freeness and hence the student-optimal stable mechanism more frequently satisfies local converse consistency than local consistency. Finally, in order for the student-optimal stable mechanism to be globally conversely consistent, one again has to impose unit acyclicity on the priority structure. Hence, unit acyclicity is a necessary and sufficient condition on the priority structure for the student-optimal stable mechanism to satisfy global consistency or global converse consistency.

Optimal coexistence of long-term and short-term contracts in labor Markets

We consider a market where firms hire workers to run their projects and such projects differ in profitability. At any period, each firm needs two workers to successfully run its project: a junior agent, with no specific skills, and a senior worker, whose effort is not verifiable. Senior workers differ in ability and their competence is revealed after they have worked as juniors in the market. We study the length of the contractual relationships between firms and workers in an environment where the matching between firms and workers is the result of market interaction. We show that, despite in a one-firm-one-worker set-up long-term contracts are the optimal choice for firms, market forces often induce firms to use short-term contracts. Unless the market only consists of firms with very profitable projects, firms operating highly profitable projects offer short-term contracts to ensure the service of high-ability workers and those with less lucrative projects also use short-term contracts to save on the junior workers' wage. Intermediate firms may (or may not) hire workers through long-term contracts.

Modelos de viento aplicados a la mejora en la predicción de incendios forestales

La predicción de incendios forestales es uno de los grandes retos de la comunidad científica debido al impacto medioambiental, humano y económico que tienen en la sociedad. El comportamiento de este fenómeno es difícil de modelar debido a la gran cantidad de variables que intervienen y la dificultad que implica su correcta medición. Los simuladores de fuego son herramientas muy útiles pero, actualmente, los resultados que obtenemos tienen un alto grado de imprecisión. Desde nuestro grupo se ha trabajado en la predicción en dos etapas, donde antes de realizar cualquier predicción, se incorpora una etapa de ajuste de los parámetros de entrada para obtener mejores predicciones. Pese a la mejora que supone este nuevo paradigma de predicción, las simulaciones sobre incendios reales tienen un alto grado de error por el efecto de las condiciones meteorológicas que, usualmente, varían de manera notable durante el transcurso de la simulación. Uno de los factores más determinantes en el comportamiento de un incendio, junto con las características del terreno, es el viento. Los modelos de predicción son extremadamente sensibles al cambio en los componentes de dirección y velocidad del viento por lo que cualquier mejora que podamos introducir para mejorar la calidad de estas componentes influye directamente en la calidad de la predicción. Nuestro sistema de predicción utiliza la dirección y velocidad del viento de forma global en todo el terreno, y lo que proponemos con este trabajo es introducir un modelo de vientos que nos permita generar vientos locales en todas las celdas en las que se divide el terreno. Este viento local dependerá del viento general y de las características del terreno de dichas celdas. Consideramos que la utilización de un viento general no es suficiente para realizar una buena predicción del comportamiento de un incendio y hemos comprobado que la inclusión de un simulador de campo de vientos en nuestro sistema puede llegar a mejorar nuestras predicciones considerablemente. Los resultados obtenidos en los experimentos sintéticos que hemos realizado nos hacen ser optimistas, puesto que consideramos que la inclusión de componentes de viento locales permitirá mejorar nuestras predicciones en incendios reales.

Anàlisi de la modelització hidrològica com a eina per la predicció d'inundacions i la gestió del territori

Les inundacions són actualment les catàstrofes naturals més recurrents i les que generen un major nombre de danys i víctimes arreu del món. L'ocupació de les zones inundables a les lleres del riu és la causa principal d’aquests desastres naturals. En aquest article es descriu la realització de models hidrològics com a mecanisme per la predicció d’inundacions i la gestió del territori. S’han estudiat les conques de la Riera de Santa Coloma (Catalunya) i del riu San Francisco (Guatemala) mitjançant els programes HEC-HMS i HEC-RAS, dels quals s’avalua la seva capacitat com eina per a la gestió del territori. S’ha analitzat l’efecte de la urbanització en el risc d’inundació en el cas de la Riera de Santa Coloma en base a la previsió del Plà d’Ordenament Urbanístic Municipal. S’han determinat les zones inundables resultants de episodis de precipitació extrems al Riu San Francisco per als episodis de les tempestes Stan(2005) i Agatha(2010).

Analisi espacial del canvi climàtic al continent europeu. Incerteses i exposicions principals

El canvi climàtic del segle XXI és una realitat, hi ha moltes evidències científiques que indiquen que l’escalfament del sistema climàtic és inequívoc. Malgrat això, també hi ha moltes incerteses respecte els impactes que pot comportar aquest canvi climàtic global. L’objectiu d’aquest projecte és estudiar la possible evolució futura de tres variables climàtiques, que són el rang de la temperatura diürna a prop de la superfície (DTR), la temperatura mitjana a prop de la superfície (MT) i la precipitació mensual (PL_mes) i valorar l’exposició que poden experimentar diferents cobertes del sòl i diferents regions biogeogràfiques del continent europeu davant d’aquests possibles patrons de canvi. Per això s’han utilitzat Models Climàtics Globals que fan projeccions de variables climàtiques que permeten preveure el possible clima futur. Mitjançant l’aplicatiu informàtic Tetyn s’han extret els paràmetres climàtics dels conjunts de dades del Tyndall Centre for Climate Change Research, del futur (TYN SC) i del passat (CRU TS). Les variables obtingudes s’han processat amb eines de sistemes d’informació geogràfica (SIG) per obtenir els patrons de canvi de les variables a cada coberta del sòl. Els resultats obtinguts mostren que hi ha una gran variabilitat, que augmenta amb el temps, entre els diferents models climàtics i escenaris considerats, que posa de manifest la incertesa associada a la modelització climàtica, a la generació d’escenaris d’emissions i a la naturalesa dinàmica i no determinista del sistema climàtic. Però en general, mostren que les glaceres seran una de les cobertes més exposades al canvi climàtic, i la mediterrània, una de les regions més vulnerables

Estrategies per la inversió tridimensional de dades magnetotel.lúriques:

Projecte de recerca elaborat a partir d’una estada a la Dublin Institute for Advanced Studies, Irlanda, entre setembre i desembre del 2009.En els últims anys s’ha realitzat un important avanç en la modelització tridimensional en magnetotel•lúrica (MT) gracies a l'augment d’algorismes d’inversió tridimensional disponibles. Aquests codis utilitzen diferents formulacions del problema (diferències finites, elements finits o equacions integrals), diverses orientacions del sistema de coordenades i, o bé en el conveni de signe, més o menys, en la dependència temporal. Tanmateix, les impedàncies resultants per a tots els valors d'aquests codis han de ser les mateixes una vegada que es converteixen a un conveni de signe comú i al mateix sistema de coordenades. Per comparar els resultats dels diferents codis hem dissenyat models diferents de resistivitats amb estructures tridimensional incrustades en un subsòl homogeni. Un requisit fonamental d’aquests models és que generin impedàncies amb valors importants en els elements de la diagonal, que no són menyspreables. A diferència dels casos del modelització de dades magnetotel.lúriques unidimensionals i bidimensionals, pel al cas tridimensional aquests elements de les diagonals del tensor d'impedància porten informació sobre l'estructura de la resistivitat. Un dels models de terreny s'utilitza per comparar els diferents algoritmes que és la base per posterior inversió dels diferents codis. Aquesta comparació va ser seguida de la inversió per recuperar el conjunt de dades d'una estructura coneguda.

Educational pathways of minority youth in Norway: intersections of gender, ethnicity and social class

Educational aspirations during lower secondary school and choice of upper secondary education are important for young people’s future trajectories into higher education and labour market positions. In line with ideas about reflexive, autonomous individuals (Giddens, 1991), choice of education is often represented as a young person’ individual decision, and educational guidance as aimed at discovering what ‘fits’ an individual’s personality, interests and abilities. Educational aspirations and choices are also social patterns that are reproduced. Some population categories represent exceptions from expected patterns of social reproduction of educational level and professions. In several countries, one such category is young people from families with migration experiences (Lauglo, 2000; Modood, 2004). In Norway, students have a legal right to non-compulsory upper secondary schooling and 96 percent of the students continue from lower to upper secondary school. In spite of positive developments regarding minority youths’ completion of upper secondary and higher education in later years, studies still persistently show lower educational attainment among minority youth, particularly among boys (Fekjaer, 2006). However, in lower secondary school, minor ity youth tend to have markedly higher educational aspirations and stronger learning motivation than their majority peers, as well as greater effort in school and strong adherence to school values (Lauglo, 2000) despite lower educational attainment or lower socio-economic backgrounds. In addition, gender differences in educational aspirations seem to be smaller among minority youth. The principal objective of the study in progress that will be presented in this paper, is to describe how processes relating to gendered, ethnic and class-based identities influence young people’s educational choices. The study is undertaken as a PhD project in social anthropology. The methodological approach is ethnographic longitudinal fieldwork in two multicultural lower secondary schools in Oslo. The study is part of a larger project that also include quantitative analyses of longitudinal data covering 9th graders in Oslo 2006 through four data collections during lower and upper secondary school.

Immigration and the school system

Immigration is an important problem in many societies, and it has wide-ranging effects on the educational systems of host countries. There is a now a large empirical literature, but very little theoretical work on this topic. We introduce a model of family immigration in a framework where school quality and student outcomes are determined endogenously. This allows us to explain the selection of immigrants in terms of parental motivation and the policies which favor a positive selection. Also, we can study the effect of immigration on the school system and how school quality may self-reinforce immigrants' and natives' choices.

Two necessary conditions for strategy-proofness: on what domains are they also sufficient?

A social choice function may or may not satisfy a desirable property depending on its domain of definition. For the same reason, different conditions may be equivalent for functions defined on some domains, while different in other cases. Understanding the role of domains is therefore a crucial issue in mechanism design. We illustrate this point by analyzing the role of different conditions that are always related, but not always equivalent to strategy-proofness. We define two very natural conditions that are necessary for strategy-proofness: monotonicity and reshuffling invariance. We remark that they are not always sufficient. Then, we identify a domain condition, called intertwinedness, that ensures the equivalence between our two conditions and that of strategy-proofness. We prove that some important domains are intertwined: those of single-peaked preferences, both with public and private goods, and also those arising in simple models of house allocation. We prove that other necessary conditions for strategy-proofness also become equivalent to ours when applied to functions defined on intertwined domains, even if they are not equivalent in general. We also study the relationship between our domain restrictions and others that appear in the literature, proving that we are indeed introducing a novel proposal.

Predicting random level and seasonality of hotel prices. A structural equation growth curve approach

This article examines the effect on price of different characteristics of holiday hotels in the sun-and-beach segment, under the hedonic function perspective. Monthly prices of the majority of hotels in the Spanish continental Mediterranean coast are gathered from May to October 1999 from the tour operator catalogues. Hedonic functions are specified as random-effect models and parametrized as structural equation models with two latent variables, a random peak season price and a random width of seasonal fluctuations. Characteristics of the hotel and the region where they are located are used as predictors of both latent variables. Besides hotel category, region, distance to the beach, availability of parking place and room equipment have an effect on peak price and also on seasonality. 3- star hotels have the highest seasonality and hotels located in the southern regions the lowest, which could be explained by a warmer climate in autumn

Mixing compositions and other scales

Theory of compositional data analysis is often focused on the composition only. However in practical applications we often treat a composition together with covariableswith some other scale. This contribution systematically gathers and develop statistical tools for this situation. For instance, for the graphical display of the dependenceof a composition with a categorical variable, a colored set of ternary diagrams mightbe a good idea for a first look at the data, but it will fast hide important aspects ifthe composition has many parts, or it takes extreme values. On the other hand colored scatterplots of ilr components could not be very instructive for the analyst, if theconventional, black-box ilr is used.Thinking on terms of the Euclidean structure of the simplex, we suggest to set upappropriate projections, which on one side show the compositional geometry and on theother side are still comprehensible by a non-expert analyst, readable for all locations andscales of the data. This is e.g. done by defining special balance displays with carefully-selected axes. Following this idea, we need to systematically ask how to display, explore,describe, and test the relation to complementary or explanatory data of categorical, real,ratio or again compositional scales.This contribution shows that it is sufficient to use some basic concepts and very fewadvanced tools from multivariate statistics (principal covariances, multivariate linearmodels, trellis or parallel plots, etc.) to build appropriate procedures for all these combinations of scales. This has some fundamental implications in their software implementation, and how might they be taught to analysts not already experts in multivariateanalysis

Virus infection speeds: theory versus experiment

In order to explain the speed of Vesicular Stomatitis Virus VSV infections, we develop a simple model that improves previous approaches to the propagation of virus infections. For VSV infections, we find that the delay time elapsed between the adsorption of a viral particle into a cell and the release of its progeny has a veryimportant effect. Moreover, this delay time makes the adsorption rate essentially irrelevant in order to predict VSV infection speeds. Numerical simulations are in agreement with the analytical results. Our model satisfactorily explains the experimentally measured speeds of VSV infections

Spatial Analysis of Cell Composition Data

A version of Matheron’s discrete Gaussian model is applied to cell composition data.The examples are for map patterns of felsic metavolcanics in two different areas. Q-Qplots of the model for cell values representing proportion of 10 km x 10 km cell areaunderlain by this rock type are approximately linear, and the line of best fit can be usedto estimate the parameters of the model. It is also shown that felsic metavolcanics in theAbitibi area of the Canadian Shield can be modeled as a fractal

Thermodynamics and log–contrast analysis in fluid geochemistry

There are two principal chemical concepts that are important for studying the naturalenvironment. The first one is thermodynamics, which describes whether a system is atequilibrium or can spontaneously change by chemical reactions. The second main conceptis how fast chemical reactions (kinetics or rate of chemical change) take place wheneverthey start. In this work we examine a natural system in which both thermodynamics andkinetic factors are important in determining the abundance of NH+4 , NO−2 and NO−3 insuperficial waters. Samples were collected in the Arno Basin (Tuscany, Italy), a system inwhich natural and antrophic effects both contribute to highly modify the chemical compositionof water. Thermodynamical modelling based on the reduction-oxidation reactionsinvolving the passage NH+4 -& NO−2 -& NO−3 in equilibrium conditions has allowed todetermine the Eh redox potential values able to characterise the state of each sample and,consequently, of the fluid environment from which it was drawn. Just as pH expressesthe concentration of H+ in solution, redox potential is used to express the tendency of anenvironment to receive or supply electrons. In this context, oxic environments, as thoseof river systems, are said to have a high redox potential because O2 is available as anelectron acceptor.Principles of thermodynamics and chemical kinetics allow to obtain a model that oftendoes not completely describe the reality of natural systems. Chemical reactions may indeedfail to achieve equilibrium because the products escape from the site of the rectionor because reactions involving the trasformation are very slow, so that non-equilibriumconditions exist for long periods. Moreover, reaction rates can be sensitive to poorly understoodcatalytic effects or to surface effects, while variables as concentration (a largenumber of chemical species can coexist and interact concurrently), temperature and pressurecan have large gradients in natural systems. By taking into account this, data of 91water samples have been modelled by using statistical methodologies for compositionaldata. The application of log–contrast analysis has allowed to obtain statistical parametersto be correlated with the calculated Eh values. In this way, natural conditions in whichchemical equilibrium is hypothesised, as well as underlying fast reactions, are comparedwith those described by a stochastic approach

Compositional Time Series: An Application

The composition of the labour force is an important economic factor for a country.Often the changes in proportions of different groups are of interest.I this paper we study a monthly compositional time series from the Swedish LabourForce Survey from 1994 to 2005. Three models are studied: the ILR-transformed series,the ILR-transformation of the compositional differenced series of order 1, and the ILRtransformationof the compositional differenced series of order 12. For each of thethree models a VAR-model is fitted based on the data 1994-2003. We predict the timeseries 15 steps ahead and calculate 95 % prediction regions. The predictions of thethree models are compared with actual values using MAD and MSE and the predictionregions are compared graphically in a ternary time series plot.We conclude that the first, and simplest, model possesses the best predictive power ofthe three models