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.

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. Dans cet article, nous cherchons à développer un modèle par lequel le salaire d'un travailleur est fonction de ses qualifications. Le marché ainsi que le travailleur sont au préalable dans l'incertitude quant à certaines de ces qualifications. L'endogénéité à la fois des changements de salaire et des décisions de changements du secteur d'affiliation résulte du processus d'apprentissage relié aux qualifications du travailleur. Nous montrons ensuite comment le modèle peut être estimé par les méthodes des variables instrumentales non-linéaires. Nous appliquons notre méthodologie à l'étude des salaires et de l'allocation des travailleurs aux différentes occupations et industries. Nous trouvons que les secteurs à salaires élevés emploient des travailleurs ayant davantage de qualifications et que ces secteurs rémunèrent ces qualifications à un taux supérieur relativement aux secteurs à faibles salaires. Les estimés des rendements associés aux qualifications qui ne tiennent pas compte du fait que les rendements diffèrent d'un secteur à un autre sont par conséquent erronés. Nous proposons enfin d'autres applications possibles de notre méthodologie.comparative advantage, learning, non-linear instrumental variables, avantages comparés, apprentissage, variables instrumentales

A magnetic liquid deformable mirror for high stroke and low order axially symmetrical aberrations

We present a new class of magnetically shaped deformable liquid mirrors made of a magnetic liquid (ferrofluid). Deformable liquid mirrors offer advantages with respect to deformable solid mirrors: large deformations, low costs and the possibility of very large mirrors with added aberration control. They have some disadvantages (e.g. slower response time). We made and tested a deformable mirror, producing axially symmetrical wavefront aberrations by applying electric currents to 5 concentric coils made of copper wire wound on aluminum cylinders. Each of these coils generates a magnetic field which combines to deform the surface of a ferrofluid to the desired shape. We have carried out laboratory tests on a 5 cm diameter prototype mirror and demonstrated defocus as well as Seidel and Zernike spherical aberrations having amplitudes up to 20 microns, which was the limiting measurable amplitude of our equipmentComment: To appear in Optics Expres

Stratégie empirique d'un système de ressources en eau: l'exemple d'un périmètre irrigué en zone semi-aride

Une règle de gestion interannuelle empirique est définie pour gérer un barrage en éte d'un périmètre d'irrigation en zone semi-aride. La structure de la règle est construite à partir de l'observation de la gestion pratiquée actuellement sur le terrain. Les paramètres de cette règle sont ajustés de façon à minimiser le cumul moyen des écarts entre demandes et ressources des séries générées après une étude fréquentielle des aléas hydrauliques.Dans le cas du périmètre lié au barrage de Ghézala, cette méthode permet de répartir au mieux les risques de défaillance sur l'année et d'adapter la stratégie d'irrigation aux phases végétatives critiques des cultures.Les avantages et les inconvénients de cette approche empirique par rapport aux techniques de programmation dynamique sont finalement discutés et mis en relief pour les systèmes de gestion des ressources en zones semi-arides.An empirical inter-annual management rule is defined to operate a reservoir for irrigation use in a semi-arid zone. Management rules for hydro-agricultural systems in semi-arid zones consist of establishing a trade-off between the total quantity of water to be distributed for summer irrigation and the indispensable volumes to be held in store for the raising of future seedlings during the following autumn. In less arid countries, water is sufficiently cheap so that it is economically profitable to deliver great quantities to sustain maximum agricultural output. Indeed, water is sufficiently abundant so that it is generally useless to conserve reserves for the next year. Such an inter-annual compromise is not the norm for water resources managers in humid climates. For instance, the two consecutive dry summers in Europe in 1989 and 1990 would not have had the same impact on the agricultural economy in the semi-arid zones. There rules of management would have been adapted to obtain of an inter-annual balance.Numerous mathematical methods have been used in the domain of water resource systems management, sustained by the constant increase of computer performances. Reservoir management issues have consequently been widely studied and solved both from the viewpoint of the art of modeling and the methods of optimization. Nevertheless it is necessary to ensure that the mathematically correct optimal solution is also implementable when one leaves the computer screen to get back to the field: the success of this passage depends largely on the ability of the objective function to quantify the economic stakes encoded in the choice of a management rule. Except for hydropower generation, the various uses of a dam are most unlikely to be integrated into an aggregative utility function under the form of separable additive criteria.A way to bypass the inherent difficulties in the design of a realistic objective quantification is to restrict the search of the operation rule to the class of strategies compatible with the commonly met attitude of semi-arid water resources system managers. The structure of the rule is designed from the observation of the effective management presently in operation. Its logic fits to the following reasoning:- when the reservoir level is low, as soon as it starts to vary, the irrigation perimeter manager will behave very prudently. i.e. by reducing the proportion of the allocated irrigation water to the amount in store; - for average working conditions, the system manager will deliver a constant fraction of the available reservoir storage for irrigation; - when the quantity stored in the dam is high, the manager tends to satisfy completely the irrigation demand. The parameters of this piecewise linear rule are adjusted so as to minimize the probability of discrepancy between demands and resources on series obtained after a brief hydrological study.The Ghezala perimeter in Tunisia, which is situated in the watershed of lake Ichkeul, has been taken as a case study. With a thousand hectares of irrigated crops to be satisfied from an upstream 12 hm3 reservoir, and 625 mm rain per year on the watershed with a strong seasonal dependence, the Ghezala perimeter is quite representative of Tunisian irrigation practices. The method developed in this paper allows to balance the risk of failure all over the year and to adapt the strategy of irrigation according to critical vegetative phases of the crops. A statistical analysis of possible deficits according to the improved rule reveals that the method improves the performance of irrigation and decreases its sensitivity to the critical vegetative phases that are of critical importance in agriculture in semi-arid zones.The management model is unsophisticated but its objective is to provide elements of appreciation to the manager of the irrigated perimeter. Each component of the model can be improved and more finely described. Advantages and drawbacks of this empirical approach with regards to more conventional dynamic programming techniques are finally discussed and underlined for water resource systems with specific attention to their management in semi-arid zones

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

Communication Strategy: Does the Two-Step Still Work?

Much of our work in cooperative extension deals with the transfer of information or technology to clientele. Therefore, it is extremely important that we use the most effective and efficient means possible in carrying out this task

Government Sponsored Health Care: A Cluster Profile of Supporters and Nonsupporters

While there has been a great deal of information revealing the public\u27s dissatisfaction with our current health care system, there is little detailed analysis of these attitudes, and of the individuals who are most likely to support or reject such a system. This becomes more and more important as health policy debates shift toward a questioning of the viability of the current health care system and possible alterations to that system. In this paper we use cluster analytic methods on data collected from a public opinion survey of Louisiana residents to develop profiles of those people who support and who reject government-sponsored health care for all citizens. We then use these profiles to develop informed strategies for use by sociologists to impact health care policy. Much of the literature on attitudes toward human resource spending were confirmed by the multivariate analysis we performed. However, the cluster analysis illuminated the true diversity that exists. Quite often, rather weak statistical relationships tend to be overgeneralized. In attempting to develop these profiles, the cluster analysis allowed us to regain the diversity in a comprehensive fashion. We found that there are clear groupings of both supporters and nonsupporters, but probably of greater importance is that there is more similarity between supporters and nonsupporters than distinctiveness