Comparative Advantage, Learning, and Sectoral Wage Determination

We develop a model in which a worker's skills determine the worker's current wage and sector. Both the market and the worker are initially uncertain about some of the worker's skills. Endogenous wage changes and sector mobility occur as labor-market participants learn about these unobserved skills. We show how the model can be estimated using non-linear instrumental-variables techniques. We then apply our methodology to study the wages and allocation of workers across occupations and across industries. For both occupations and industries, we find that high-wage sectors employ high-skill workers and offer high returns to workers' skills. Estimates of these sectoral wage differences that do not account for sector-specific returns are therefore misleading. We also suggest further applications of our theory and methodology.

Gestion stratégique d'un système de ressources en eau : l'exemple du système NESTE

L'article que nous proposons s'inscrit dans le cadre des problèmes d'optimisation bimensionnelle (irrigation & salubrité) des ressources en eau durant la période d'étiage. Sur le cas du système NESTE, la résolution est effectuée selon deux approches :- un modèle de programmation dynamique avec état de dimension deux (niveau des réserves, niveau dans la rivière) où, dans la solution numérique, les variables sont discrétisées;- un modèle « synthétique » où l'on calcule une probabilité de non dépassement caractérisant l'état hydrique des ressources du système. Une règle empirique permet d'associer à cette grandeur une décision de consigne à effectuer.Les résultats numériques sont comparés sur une série de chroniques historiques. Les avantages et les inconvénients de chacune des deux approches sont mis en lumière sur le cas réel du système NESTE.This paper deals with bicreteria (irrigation & water quality) weekly operation of a water resource system during dry period. Two ways of handling the problem are assessed and compared on a real case study :- a stochastic dynamic programming modal with a two dimensional state (reservoirs level, river level) that is numerically solved by discretization ;- a more « synthetic » model where the state is expressed in term of a tail aera probability related to the consumption of all the present water resources in the future. A practical decision rule is based upon the associated critical value.Numerical results are plotted on historical varies for both methods.From the present application to the NESTE system, the conclusions are :1) Both procedures allow the system manager to formulate operating strategies in a rational way :- An operating rule can be derived to allocate water so as to meet a combination of the various objectives. It is expressed as a feedback law linking what we know from the state of the system to how we control its evolution.- Both methods need a parameter to be set up by stochastic simulation.- They give close results on the basis of the past data and can be conveniently proposed to system managers.2) The system analysis approach is based on stochastic dynamic programming. If can be efficiently used to derive optimal feedback ruses of operation and can routinely deal with complex decisions such as limiting irrigation when a shortage is to occur or take the risk to keep going and decrease output targets for water quality management. At the same time, this procedure entails heavy computing time, uneasy interpretation of the weighting coefficient between irrigation and water quality objectives, and a rather artificial elicitation of the global compromise.Such an approach is very well fit for simulation because it is composed of elementary blocks that are gathered in a transition relationship to describe the system's dynamic evolution. This approach also provides a means to get an optimal policy as long as the system manager accepts the necessity to formulate an objective function consistently with dynamic programming (i. e. stages are separable and additive). Of course this optimal allocation should be carefully examined because of modal uncertainties influencing both the system response and the hydrological behaviour.3) The synthetic method may appear more attractive from the engineering point of view for the following reasons :- the state is easily interpreted in terms of « dry year with a return period of 10, normal year, exceptionally wet year » and so on. The trade-off coefficient is the volume one wants to keep in the reservoirs at time T for a wet year. Consequently if the parameter is chosen with « good sense », no optimization scheme is needed.- there is no computation except a mass balance equation and a normal probability law adjustment which is very easy because it deals with cumulative quantities.4) Such models are designed to serve only as multicriterion decision making aids. In very dry days such as occurred in summer 1976 or 1989 in France they cannot create additional water resources... still, they can help the system manager by constant up dated multidimensional estimation of the risks that may be encountered when following different operation rotes. In the case of the NESTE system, a real-scale experiment began in 1989: in real time operation, both models worked on line as decision-making supports, and the system manager made a thorough study of the hydrological conditions when the two approaches did not agree on the same stragegy for the following week

The Concept of Social Exclusion and Rural Development Policy

Since the early 1970s rural research and public education programs have been intensified in efforts to improve living conditions and strengthen community life in rural America (Southern Perspectives 2000). During much of the 1990s, the nation, including the rural South, experienced a growing economy, a booming stock market, and declining unemployment rates (Gibbs 2001). However, many serious social problems traditionally associated with the rural South remain to this day (Gibbs 2001). This paper introduces the concept of social exclusion, used extensively in European countries and now part of the European Union\u27s official lexicon. Social exclusion is defined as the process and the resulting condition in which specific social entities are fully or partially prevented from acquiring the basic necessities of life. Further components are that it is seen as a product of the social system, not an individual attribute, and that it is multi-dimensional and dynamic in time and space. It is argued that the concept of social exclusion should be incorporated into rural development policy discourse in the United States. This would aid in countering the predominant pattern of neglect in rural development policies and programs in addressing the persistent problems which exist

Changing the Health Care System: Opinions of Rural and Urban Residents

This paper examines the opinions of rural and urban residents toward a full health care system provided by the government. The data used in the study come from a statewide poll conducted by the Louisiana Department of Health and Hospitals. Because of a greater need for health care reform in rural areas, it was assumed that rural Louisianians might be more supportive of a government health care system than their urban counterparts. However, analysis of the data indicates that a person\u27s residence had no statistically significant effect on attitudes toward government sponsored health care

Understanding the Concentration Dependence of Viral Capsid Assembly Kinetics - the Origin of the Lag Time and Identifying the Critical Nucleus Size

The kinetics for the assembly of viral proteins into a population of capsids can be measured in vitro with size exclusion chromatography or dynamic light scattering, but extracting mechanistic information from these studies is challenging. For example, it is not straightforward to determine the critical nucleus size or the elongation time (the time required for a nucleated partial capsid to grow completion). We show that, for two theoretical models of capsid assembly, the critical nucleus size can be determined from the concentration dependence of the assembly reaction half-life and the elongation time is revealed by the length of the lag phase. Furthermore, we find that the system becomes kinetically trapped when nucleation becomes fast compared to elongation. Implications of this constraint for determining elongation mechanisms from experimental assembly data are discussed.Comment: Submitted to Biophysical Journa

Wavefront correction with a ferrofluid deformable mirror: experimental results and recent developments

We present the research status of a deformable mirror made of a magnetic liquid whose surface is actuated by a triangular array of small current carrying coils. We demonstrate that the mirror can correct a 11 microns low order aberrated wavefront to a residual RMS wavefront error 0.05 microns. Recent developments show that these deformable mirrors can reach a frequency response of several hundred hertz. A new method for linearizing the response of these mirrors is also presented.Comment: To appear in "Ground-based and Airborne Instrumentation for Astronomy II" SPIE conference, Marseille, 23-28 June 200